Optical head, optical element with diffraction grating, optical disc device and information processing device

ABSTRACT

An optical head includes a blue-violet laser light source for emitting a blue-violet laser beam, a mirror with a diffraction grating for transmitting and reflecting the blue-violet laser beam at a predetermined ratio, and a front monitor sensor for receiving transmitted light or reflected light from the mirror with a diffraction grating and creating an APC signal for controlling an output of the blue-violet laser light source. The mirror with a diffraction grating includes a first surface which the blue-violet laser beam enters, and a second surface facing the first surface. The first surface and the second surface are mutually parallel. A reflecting coat for transmitting and reflecting the blue-violet laser beam at a predetermined ratio is formed on the first surface, and a diffraction grating is formed on the second surface.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an optical head for recording orreproducing information to or from an information recording medium suchas an optical disc, an optical disc device comprising such an opticalhead, an information processing device comprising such an optical discdevice, and an optical element with a diffraction grating for guiding alaser beam to a photodetector that creates an automatic power controlsignal for controlling an output of a light source.

2. Background Art

An optical head for recording or reproducing information to or from aninformation recording medium such as an optical disc comprises a frontmonitor sensor for detecting a part of the laser beam that is emittedfrom the light source in order to more accurately control the output ofthe laser beam that is emitted from the light source particularly duringthe recording process. The detection signal in the front monitor sensoris an APC (Automatic Power Control) signal. The APC signal is fed backto the control unit for controlling the output of the light source. TheAPC signal is used for controlling the output of the light source sothat appropriate power that is required for the recording and/orreproduction of the information can be obtained.

Nevertheless, if the laser beam that is emitted from the light source isconverted into parallel light with, for instance, a collimator lens orthe like, the optical axis of the laser beam that is headed toward thefront monitor sensor after passing through or being reflected by theplanar beam splitter or the planar reflective mirror and the opticalaxis of the laser beam that is headed toward the front monitor sensorafter being internally reflected by the planar beam splitter or theplanar reflective mirror will mutually become parallel and causeinterference. Consequently, the APC signal in the front monitor sensorwill no longer be accurately proportionate to the output of the lightsource.

FIG. 17 is a diagram showing the condition of transmitted light andreflected light in a conventional planar beam splitter. For example, asshown in FIG. 17, parallel light P1 that is emitted from within theeffective region of the collimator lens enters a planar beam splitter105. Here, the parallel light P1 is split into reflected light R thatwas reflected by a first surface 105 a and transmitted light T1 thatpassed through the first surface 105 a and a second surface 105 b and isheaded toward the front monitor sensor. Here, parallel light P2 that wasemitted from within a different effective region of the collimator lenspasses through the first surface 105 a, and is subsequently reflected bythe second surface 105 b. Further, the parallel light P2 that wasreflected by the second surface 105 b is reflected by the first surface105 a, subsequently passes through the second surface 105 b, and isemitted as the transmitted light T2.

Here, if the optical axis of the parallel light P1 and the optical axisof the parallel light P2 are mutually parallel, and the first surface105 a and the second surface 105 b of the planar beam splitter 105 aremutually parallel, the optical axis of the transmitted light T1 and theoptical axis of the transmitted light T2 will become mutually parallel,and cause an interference in the effective region of the front monitorsensor. Thus, even if the output of the light source is changedlinearly, the APC signal that was detected with the front monitor sensorand converted into an electrical signal will not change linearly.

As described above, in an optical configuration where parallel light iscaused to enter the planar beam splitter or the planar reflective mirrorin which the light incident plane and the light emission plane areparallel, it will become difficult to accurately control the output ofthe light source.

Thus, Patent Literature 1 describes an optical pickup for inhibiting theinterference of the laser beam caused by the internal reflection withthe planar beam splitter by adopting an optical configuration in whichconverging light or diverging light enters the planar beam splitter. Inaddition, Patent Literature 1 describes an optical pickup for inhibitingthe interference of the laser beam caused by the internal reflectionwith the beam splitter by using a wedge-shaped beam splitter.

The conventional optical pickup described in Patent Literature 1 is nowexplained with reference to FIG. 18. FIG. 18 is a diagram showing aschematic configuration of a conventional optical pickup.

In FIG. 18, an optical pickup 150 is configured from first and secondlight sources 110, 120 for emitting light respectively having differentwavelengths, a planar beam splitter 125, first and second collimatorlenses 114, 124 disposed between the first and second light sources 110,120 and the planar beam splitter 125, a front monitor sensor 126, amirror 127 and an objective lens 129.

In the conventional optical pickup 150, the front monitor sensor 126 isused for detecting an APC signal. In the foregoing case, the firstcollimator lens 114 is disposed so as to convert the laser beam that isemitted as diverging light from the first light source 110 intoconverging light or diverging light. Specifically, the first collimatorlens 114 is disposed in a state of being moved to a position that iscloser to the first light source 110 or to a position that is fartheraway from the first light source 110 than a position of changing thelaser beam that is emitted from the first light source 110 to parallellight.

For example, a case of converting the laser beam that is emitted fromthe first light source 110 into diverging light with the firstcollimator lens 114 is now explained with reference to FIG. 19. FIG. 19is a diagram showing the condition of the transmitted light and thereflected light in the planar beam splitter of the conventional opticalpickup shown in FIG. 18.

In FIG. 19, the planar beam splitter 125 includes a first surface 125 awhich the laser beam that was emitted from the first light source 110enters, and a second surface 125 b facing the first surface 125 a.Diverging light Q3 that was emitted from the effective region of thefirst collimator lens 114 enters the planar beam splitter 125. Here, thediverging light Q3 is split into reflected light R that was reflected bythe first surface 125 a and transmitted light T3 that passed through thefirst surface 125 a and the second surface 125 b and is headed towardthe front monitor sensor 126. Here, diverging light Q4 that was emittedwithin a different effective region of the first collimator lens 114will be reflected by the second surface 125 b after passing through thefirst surface 125 a. In addition, the diverging light Q4 that wasreflected by the second surface 125 b is reflected by the first surface125 a, subsequently passes through the second surface 125 b, and isemitted as transmitted light T4.

Here, since the diverging light Q3 and the diverging light Q4 areemitted from different effective regions of the first collimator lens114, the optical axis of the diverging light Q3 and the optical axis ofthe diverging light Q4 will not be parallel. Accordingly, even if thefirst surface 125 a and the second surface 125 b of the planar beamsplitter 125 are mutually parallel, the optical axis of the transmittedlight T3 and the optical axis of the transmitted light T4 will not beparallel.

As described above, as a result of the collimator lens 114 beingdisposed so that the space between the first light source 110 and thefirst collimator lens 114 will be shorter than the focal length of thefirst collimator lens 114, the laser beam that is headed toward theplanar beam splitter 125 will become diverging light, and the frontmonitor sensor 126 and is disposed to receive the laser beam that isemitted from the first light source 110. Here, the optical axis of thelaser beam that passed through the planar beam splitter 125 and isheaded toward the front monitor sensor 126 and the optical axis of thelaser beam that was internally reflected two or more times in the planarbeam splitter 125 and is subsequently headed toward the front monitorsensor 126 will not be mutually parallel. Accordingly, it is possible toinhibit the interference of the laser beam in the effective region ofthe front monitor sensor 126.

With this kind of conventional optical pickup, the front monitor sensor126 is able to create an APC signal that is accurately proportionate tothe quantity of light of the first light source 110. If this APC signalis fed back to the control unit that is controlling the first lightsource 110 in order to control the output of the first light source 110,the laser beam can be emitted with a recording power possessinglinearity. Thus, upon recording information on an optical disc, thefirst light source 110 can be accurately controlled so that the laserbeam to be emitted will have the intended recording power.

Meanwhile, the second collimator lens 124 is disposed so as to convertthe laser beam that is emitted as diverging light from the second lightsource 120 into diverging light or converging light. Thus, the frontmonitor sensor 126 is also able to accurately control the second lightsource 120 so that the laser beam to be emitted will have the intendedrecording power.

The conventional optical pickup comprising the wedge-shaped beamsplitter described in Patent Literature 1 is now explained. The opticalpickup comprising the wedge-shaped beam splitter is characterized incomprising a wedge-shaped beam splitter 145 in substitute for the planarbeam splitter 125 of the optical pickup 150 shown in FIG. 18.

FIG. 20 is a diagram showing the condition of the transmitted light andthe reflected light in the wedge-shaped beam splitter of a conventionaloptical pickup. With the wedge-shaped beam splitter 145, as shown inFIG. 20, the first surface 145 a and the second surface 145 b mutuallyform a predetermined angle. The first collimator lens 114 and/or secondcollimator lens 124 is disposed so as to converge the laser beam that isemitted as diverging light from the first light source 110 and thesecond light source 120 into parallel light. With the wedge-shaped beamsplitter 145, the angle formed by the first surface 145 a and the secondsurface 145 b is decided so that interference will not occur in theeffective region of the front monitor sensor 126 due to the internalreflection occurring two or more times.

In FIG. 20, parallel light P5 that is emitted from within the effectiveregion of the first collimator lens 114 enters the wedge-shaped beamsplitter 145. Here, the parallel light P5 is split into reflected lightR that was reflected by the first surface 145 a and transmitted light T5that passed through the first surface 145 a and the second surface 145 band is headed toward the front monitor sensor 126. Parallel light P6that was emitted from within a different effective region of the firstcollimator lens 114 is reflected by the second surface 145 b afterpassing through the first surface 145 a. In addition, the parallel lightP6 that was reflected by the second surface 145 b is reflected by thefirst surface 145 a, subsequently passes through the second surface 145b, and is emitted as transmitted light T6.

Here, although the optical axis of the parallel light P5 and the opticalaxis of the parallel light P6 are mutually parallel, since the firstsurface 145 a and the second surface 145 b of the wedge-shaped beamsplitter 145 are forming a predetermined angle, the optical axis of thetransmitted light T5 and the optical axis of the transmitted light T6will not be parallel.

As described above, as a result of using the wedge-shaped beam splitter145 in which the light incident plane and the light emission plane forma predetermined angle, the laser beam that is emitted from the firstlight source 110, passes through the wedge-shaped beam splitter 145, andis subsequently headed toward the front monitor sensor 126 and the laserbeam that is headed toward the front monitor sensor 126 after beinginternally reflected at least two or more times in the wedge-shaped beamsplitter 145 will not be mutually parallel. Accordingly, it is possibleto inhibit the interference of the laser beam in the effective region ofthe front monitor sensor 126 and create an APC signal that is accuratelyproportionate to the quantity of light of the first light source 110,and accurately control the optical output of the first light source 110.

Moreover, as a result of using the wedge-shaped beam splitter 145, thelaser beam that is emitted from the second light source 120, internallyreflected once in the wedge-shaped beam splitter 145 and is subsequentlyheaded toward the front monitor sensor 126 and the laser beam that isheaded toward the front monitor sensor 126 after being internallyreflected at least three or more times in the wedge-shaped beam splitter145 will not be mutually parallel. Accordingly, it is possible toinhibit the interference of the laser beam in the effective region ofthe front monitor sensor 126 and create an APC signal that is accuratelyproportionate to the quantity of light of the second light source 120,and accurately control the optical output of the second light source120.

Meanwhile, pursuant to the practical application of a blue-violetsemiconductor laser, a Blu-ray Disc (hereinafter referred to as “BD”) asa high density, large capacity optical information recording medium(hereinafter also referred as an “optical disc”) of the same size as aCD (Compact Disc) and a DVD (Digital Versatile Disc) has been put intopractical use. The BD is an optical disc that uses a blue-violet laserlight source for emitting blue-violet light having a wavelength ofapproximately 400 nm and an objective lens having a numerical aperture(Numerical Aperture: NA) that is approximately 0.85 and records orreproduces information to or from the information recording surface inwhich the thickness of the light transmitting layer is approximately 0.1mm.

With a high density optical disc such as a BD, information will berecorded or reproduced to or from a plurality of information recordingsurfaces. However, since the thickness of the light transmitting layerfor each information recording surface is different, third orderspherical aberration will occur in accordance with the distance from theoptimal light transmitting layer thickness to the information recordingsurface on the information recording surface that deviates from theoptimal light transmitting layer thickness of the objective lens.Incidentally, the optimal light transmitting layer thickness of theobjective lens refers to the thickness of the light transmitting layerin which the third order spherical aberration will be minimal whenparallel light enters the objective lens. If the wavelength of the laserbeam is 400 nm and the NA of the objective lens is 0.85, third orderspherical aberration of approximately 100 mλ will occur for a 10 μmthickness deviation of the light transmitting layer. Thus, an opticalhead for use in this kind of optical disc generally comprises means forcorrecting the third order spherical aberration.

For example, Patent Literature 2 describes an optical head in which acollimator lens is mounted on a collimator lens actuator, and, in orderto negate the third order spherical aberration caused by the thicknessdeviation of the light transmitting layer, the collimator lens disposedbetween a light source and an objective lens is moved in the opticalaxis direction to change the divergence angle or convergent angle of thelaser beam that enters the objective lens.

With a BD, following types of discs have been put into practical use;namely, a single layer disc comprising a single information recordingsurface in which the thickness of the light transmitting layer is 100μm, and a dual layer disc comprising two information recording surfaceseach having a light transmitting layer with a thickness of 100 μm and 75μm. With an optical disc comprising a plurality of information recordingsurfaces, since the thickness of the light transmitting layer willdiffer for each information recording surface, the collimator lens needsto be moved in a wide range in order to negate the third order sphericalaberration caused by the thickness deviation of the light transmittinglayer or various errors. Specifically, the laser beam that is emittedfrom the collimator lens will be used in a broad range of the converginglight, the parallel light and the diverging light. Accordingly, with anoptical head for recording or reproducing information to or from anoptical disc including a plurality of information recording surfaces, itis not possible to adopt a configuration using the conventional planarbeam splitter, and there is no choice but to use the wedge-shaped beamsplitter that entails high production costs.

-   Patent Literature 1: Japanese Patent Application Laid-open No.    2004-5944-   Patent Literature 2: Japanese Patent Application Laid-open No.    H11-259906

SUMMARY OF THE INVENTION

The present invention was devised in order to overcome the foregoingproblems, and an object thereof is to provide an optical head, anoptical element with a diffraction grating, an optical disc device andan information processing device capable of accurately controlling thelaser power of the laser beam that is emitted from the light source.

The optical head according to one aspect of the present inventioncomprises a first light source for emitting a first laser beam having afirst wavelength λ1, a first planar optical element for transmitting andreflecting the first laser beam at a predetermined ratio, a firstobjective lens for converging the first laser beam onto an informationrecording surface of a first information recording medium, a firstphotodetector for receiving reflected light from the informationrecording surface of the first information recording medium and creatingan information signal and/or an error signal, and a second photodetectorfor receiving transmitted light or reflected light from the first planaroptical element and creating an automatic power control signal forcontrolling an output of the first light source, wherein the firstplanar optical element includes a first surface which the first laserbeam emitted from the first light source enters, and a second surfacefacing the first surface, the first surface and the second surface aremutually parallel, a reflective coat for transmitting and reflecting thefirst laser beam at a predetermined ratio is formed on the firstsurface, and a diffraction grating is formed on the second surface.

According to this configuration, the first light source emits a firstlaser beam having a first wavelength λ1, and the first planar opticalelement transmits and reflects the first laser beam at a predeterminedratio. The first objective lens converges the first laser beam onto aninformation recording surface of a first information recording medium,and the first photodetector receives reflected light from theinformation recording surface of the first information recording medium,and creates an information signal and/or an error signal. The secondphotodetector receives transmitted light or reflected light from thefirst planar optical element, and creates an automatic power controlsignal for controlling the output of the first light source. The firstplanar optical element includes a first surface which the first laserbeam emitted from the first light source enters, and a second surfacefacing the first surface, and the first surface and the second surfaceare mutually parallel. A reflective coat for transmitting and reflectingthe first laser beam at a predetermined ratio is formed on the firstsurface, and a diffraction grating is formed on the second surface.

According to the present invention, it is possible to inhibit theinterference of the laser beam in the effective region of thephotodetector for controlling the laser power of the light source, andit is thereby possible to accurately control the laser power of thelaser beam that is emitted from the light source.

The object, features and advantages of the present invention will becomeclearer based on the ensuing detailed explanation and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the schematic configuration of the opticalhead in the first embodiment of the present invention.

FIG. 2 is a diagram showing the schematic configuration of themultilayer optical disc in the first embodiment of the presentinvention.

FIG. 3 is a view showing a frame format of the configuration of theobjective lens actuator in the first embodiment of the presentinvention.

FIG. 4 is a view showing a frame format of the schematic configurationof the collimator lens actuator in the first embodiment of the presentinvention.

FIG. 5A is a diagram showing the outgoing beam when the collimator lensis in the reference position, FIG. 5B is a diagram showing the outgoingbeam when the collimator lens moves to the light source side, and FIG.5C is a diagram showing the outgoing beam when the collimator lens movesto the objective lens side.

FIG. 6 is a diagram showing the schematic configuration of the mirrorwith a diffraction grating in the first embodiment of the presentinvention.

FIG. 7 is a diagram showing the condition of the transmitted light andthe reflected light of the mirror with a diffraction grating in thefirst embodiment of the present invention.

FIG. 8 is a diagram showing the relationship between the depth of thediffraction grating, and the respective diffraction efficiencies of thetransmitted zero order light, the transmitted±first order diffractedlight, the internally reflected zero order light, and the internallyreflected±first order diffracted light.

FIG. 9 is a diagram showing the schematic configuration of the mirrorwith a diffraction grating in a modified example of the first embodimentof the present invention.

FIG. 10 is a diagram showing the schematic configuration of the opticalhead in the second embodiment of the present invention.

FIG. 11 is a diagram showing the schematic configuration of the opticalhead in the third embodiment of the present invention.

FIG. 12 is a diagram showing the schematic configuration of the opticalhead in the fourth embodiment of the present invention.

FIG. 13 is a diagram showing the schematic configuration of the opticaldisc device in the fifth embodiment of the present invention.

FIG. 14 is a diagram showing the schematic configuration of the computerin the sixth embodiment of the present invention.

FIG. 15 is a diagram showing the schematic configuration of the opticaldisc player in the seventh embodiment of the present invention.

FIG. 16 is a diagram showing the schematic configuration of the opticaldisc recorder in the eighth embodiment of the present invention.

FIG. 17 is a diagram showing the condition of the transmitted light andthe reflected light in a conventional planar beam splitter.

FIG. 18 is a diagram showing the schematic configuration of aconventional optical pickup.

FIG. 19 is a diagram showing the condition of the transmitted light andthe reflected light in the planar beam splitter of the conventionaloptical pickup shown in FIG. 18.

FIG. 20 is a diagram showing the condition of the transmitted light andthe reflected light in a wedge-shaped beam splitter of a conventionaloptical pickup.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are now explained with reference tothe attached drawings. The following embodiments are merely examplesthat embody the present invention, and are not intended to limit thetechnical scope of the present invention in any way.

First Embodiment

FIG. 1 is a diagram showing the schematic configuration of the opticalhead in the first embodiment of the present invention. FIG. 2 is adiagram showing the schematic configuration of the multilayer opticaldisc in the first embodiment of the present invention.

In FIG. 1, an optical head 30 comprises a blue-violet laser light source1, a relay lens 2, a polarizing beam splitter 3, a collimator lens 4, amirror with a diffraction grating 5, a quarter wavelength plate 6, adiffraction lens 7, an objective lens 8, an objective lens actuator 9, acollimator lens actuator 14, a detection hologram 21, a detection lens22, a light receiving element 23 and a front monitor sensor 24.

The multilayer optical disc 60 includes, as shown in FIG. 2, fourinformation recording surfaces L0 to L3. Thickness d3 of the lighttransmitting layer of the information recording surface L0 is forinstance 100 μm, thickness d2 of the light transmitting layer of theinformation recording surface L1 is for instance 83 μm, thickness d1 ofthe light transmitting layer of the information recording surface L2 isfor instance 69 μm, and thickness d0 of the light transmitting layer ofthe information recording surface L3 is for instance 55 μm.

Incidentally, the term light transmitting layer as used in thisspecification represents the layer between the information recordingsurface and the light incident plane 61. Thus, the thickness of thelight transmitting layer of the information recording surface representsthe distance from the information recording surface to the lightincident plane 61.

Incidentally, if the thickness of the light transmitting layer isincreased, third order coma aberration caused by the inclination of theoptical disc or the objective lens will increase drastically with a highdensity optical disc such as a BD having a large NA of the objectivelens. Specifically, the third order coma aberration will increase inproportion to the cube of the NA. In other words, with an optical dischaving a plurality of information recording surfaces, the maximum valueof the thickness of the light transmitting layer is decided based on theNA of the objective lens. Thus, with a multilayer high density opticaldisc comprising three or more information recording surfaces, there isno choice but to reduce the space of the information recording surfacesto be smaller than the BD and the like which have been put intopractical use.

The blue-violet laser light source 1 emits a blue-violet laser beamhaving a first wavelength λ1 (for example, approximately 405 nm). Themirror with a diffraction grating 5 transmits and reflects theblue-violet laser beam at a predetermined ratio. Moreover, the mirrorwith a diffraction grating 5 reflects the blue-violet laser beam so thatit enters substantially perpendicularly the information recordingsurface of the multilayer optical disc 60.

The objective lens 8 converges the blue-violet laser beam onto theinformation recording surface of the multilayer optical disc 60. Thelight receiving element 23 receives the reflected light from theinformation recording surface of the multilayer optical disc 60, andcreates an information signal and/or an error signal. The front monitorsensor 24 receives the transmitted light or the reflected light of themirror with a diffraction grating 5, and creates an automatic powercontrol signal for controlling the output of the blue-violet laser lightsource 1.

The collimator lens actuator 14 corrects the spherical aberration thatoccurs according to the thickness of the light transmitting layer of themultilayer optical disc 60. Specifically, the collimator lens actuator14 causes diverging light to enter the objective lens 8 upon recordingor reproducing information to or from an information recording surfaceL0 with the thickest light transmitting layer, and causes converginglight to enter the objective lens 8 upon recording or reproducinginformation to or from an information recording surface L3 with thethinnest light transmitting layer.

The operation of the optical head 30 upon recording or reproducinginformation to or from the multilayer optical disc 60 is now explained.The blue-violet laser beam having a wavelength of approximately 405 nmthat was emitted from the blue-violet laser light source 1 is convertedinto diverging light having a different NA by the relay lens 2, andenters the polarizing beam splitter 3 by S polarization. The blue-violetlaser beam that was reflected by the polarizing beam splitter 3 isconverted into approximately parallel light with the collimator lens 4,and enters the mirror with a diffraction grating 5. A part of theblue-violet laser beam that entered the mirror with a diffractiongrating 5 is reflected in the direction of the quarter wavelength plate6. The other part of the blue-violet laser beam that entered the mirrorwith a diffraction grating 5 passes through the mirror with adiffraction grating 5, and subsequently enters the front monitor sensor24. Then the output of the blue-violet laser light source 1 iscontrolled based on the output of the front monitor sensor 24.

Meanwhile, the blue-violet laser beam that was reflected by the mirrorwith a diffraction grating 5 is converted into circular polarized lightby the quarter wavelength plate 6, and thereafter passes through thediffraction lens 7. The blue-violet laser beam that passed through thediffraction lens 7 is converged as a light spot onto any one of theinformation recording surfaces L0 to L3 of the multilayer optical disc60 by the objective lens 8.

The blue-violet laser beam that is reflected by the predeterminedinformation recording surface of the multilayer optical disc 60 passesthrough the objective lens 8 and the diffraction lens 7 once again, isconverted into linear polarized light that is different from the outwardpath by the quarter wavelength plate 6, and subsequently reflected bythe mirror with a diffraction grating 5. The blue-violet laser beam thatis reflected by the mirror with a diffraction grating 5 passes throughthe collimator lens 4, and enters the polarizing beam splitter 3 by Ppolarization. The blue-violet laser beam that passed through thepolarizing beam splitter 3 is guided to the light receiving element 23via the detection hologram 21 and the detection lens 22. The blue-violetlaser beam that was detected with the light receiving element 23 issubject to photoelectric conversion. The signal created by photoelectricconversion is operated with the control unit described later, and afocus error signal for following the surface fluctuation of themultilayer optical disc 60 and a tracking error signal for following theeccentricity of the multilayer optical disc 60 are created.

Incidentally, in the first embodiment, the blue-violet laser beamcorresponds to an example of the first laser beam and the laser beam,the blue-violet laser light source 1 corresponds to an example of thefirst light source, the mirror with a diffraction grating 5 correspondsto an example of the first planar optical element and the opticalelement with a diffraction grating, the multilayer optical disc 60corresponds to an example of the first information recording medium, theobjective lens 8 corresponds to an example of the first objective lens,the light receiving element 23 corresponds to an example of the firstphotodetector, the front monitor sensor 24 corresponds to an example ofthe second photodetector and the photodetector, and the collimator lensactuator 14 corresponds to an example of the spherical aberrationcorrection unit.

The detection of the focus error signal and the detection of thetracking error signal in the optical head of the first embodiment arenow explained.

The focus error signal for following the surface fluctuation of themultilayer optical disc 60 is detected using the so-called astigmatismmethod or the like of detecting the focal spot provided by the detectionlens 22 based on astigmatism with a quartered light receiving pattern inthe light receiving element 23.

Meanwhile, the tracking error signal for following the eccentricity ofthe multilayer optical disc 60 is created by detecting zero order lightand ±first order diffracted light which are created upon passing throughthe detection hologram 21 in a predetermined light receiving area of thelight receiving element 23. Consequently, it is possible to inhibit thevariation in the tracking error signal that occurs when there isvariation in the position, width and depth of the groove of theinformation track that is formed on the multilayer optical disc 60, andthe variation in the tracking error signal that occurs as a result ofinformation being recorded on the information track and the reflectancebeing changed thereby. It is also possible to avoid unnecessary light(stray light) that is reflected by an information recording surface thatis different from the information recording surface to be subject torecording or reproduction from entering the light receiving area inwhich the tracking error signal is to be detected.

The detection of the focus error signal and the tracking error signal isnot limited to the foregoing detection methods and, for example, it isalso possible to use a differential push-pull method (DPP method) or thelike that uses a main beam and a sub beam that are created by thediffraction grating for detecting the tracking error signal.

The objective lens actuator of this embodiment is now explained. FIG. 3is a view showing a frame format of the configuration of the objectivelens actuator in the first embodiment of the present invention.

The objective lens actuator 9 drives the objective lens 8 in a biaxialdirection so that that the light spot will follow the information trackof the rotating multilayer optical disc 60 with the focus error signaland the tracking error signal.

As shown in FIG. 3, an objective lens holder 9 b (moving part) forretaining the objective lens 8 is supported with a plurality ofsuspension wires 9 a. The objective lens actuator 9 drives the objectivelens 8 in the focus direction FD and the tracking direction TD using thefocus error signal and the tracking error signal so that the light spotwill follow the information track of the rotating multilayer opticaldisc 60.

In addition to the displacement of the focus direction FD and thetracking direction TD, the objective lens actuator 9 is also able toincline the objective lens 8 in the radial direction RD of themultilayer optical disc 60.

The collimator lens actuator in the first embodiment is now explained.The collimator lens 4 is able to move in the optical axis direction ofthe collimator lens 4 by the collimator lens actuator 14.

FIG. 4 is a view showing a frame format of the schematic configurationof the collimator lens actuator 14 in the first embodiment of thepresent invention. In FIG. 4, the collimator lens actuator 14 comprisesa stepping motor 72, a screw shaft 73, a principal shaft 74, a secondaryshaft 75 and a lens holder 76. As a result of driving the stepping motor72 and rotating the screw shaft 73, the lens holder 76 holding thecollimator lens 4 moves in the optical axis direction of the collimatorlens 4 along the principal shaft 74 and the secondary shaft 75.

FIG. 5A is a diagram showing the outgoing beam when the collimator lensis in the reference position, FIG. 5B is a diagram showing the outgoingbeam when the collimator lens moves to the light source side, and FIG.5C is a diagram showing the outgoing beam when the collimator lens movesto the objective lens side.

As shown in FIG. 5A, when the collimator lens 4 is in the referenceposition, the outgoing beam of the collimator lens 4 will beapproximately parallel light. Meanwhile, as shown in FIG. 5B, as aresult of moving the collimator lens 4 from the reference position tothe light source side, the outgoing beam of the collimator lens 4becomes diverging light, and it is possible to correct the third orderspherical aberration that occurs when the light transmitting layer ofthe multilayer optical disc 60 becomes thick.

Meanwhile, as shown in FIG. 5C, as a result of moving the collimatorlens 4 from the reference position to the objective lens side, theoutgoing beam of the collimator lens 4 becomes converging light, and itis possible to correct the third order spherical aberration that occurswhen the light transmitting layer of the multilayer optical disc 60becomes thin. Specifically, in the multilayer optical disc 60 comprisinga plurality of information recording surfaces, it is possible to correctthe third order spherical aberration by moving the collimator lens 4according to the thickness of the light transmitting layer of therespective information recording surfaces.

Incidentally, the configuration of the collimator lens actuator 14 formoving the collimator lens 4 in the optical axis direction is notlimited to the configuration using the stepping motor 72 as shown inFIG. 4 and, for example, it may also be any configuration such as anactuator that is driven with a magnetic circuit or a piezoelectricelement. With the configuration using the stepping motor 72 shown inFIG. 4, the system can be simplified since it is not necessary tomonitor the position of the optical axis direction of the collimatorlens 4. Meanwhile, since the actuator that is driven with a magneticcircuit or a piezoelectric element has a small drive part, it issuitable for miniaturizing the optical head.

The objective lens of in the first embodiment is now explained. Thedesign conditions of the objective lens 8 in the optical head 30 of thefirst embodiment are as follows. Specifically, the design wavelength is405 nm, the design light transmitting layer thickness is 80 μm, thefocal length is 1.3 mm, the numerical aperture (NA) is 0.855, and theworking distance is 0.3 mm. The design light transmitting layerthickness represents the thickness of the light transmitting layer inwhich the third order spherical aberration will be minimal when parallellight enters the objective lens.

The objective lens 8 of the first embodiment has a design lighttransmitting layer thickness of 80 μm. Thus, when collecting light onthe information recording surface L0 in which the thickness of the lighttransmitting layer is 100 μm and the information recording surface L1 inwhich the thickness of the light transmitting layer is 83 μm, diverginglight is caused to enter the objective lens 8 by moving the collimatorlens 4 to the light source side. Consequently, it is possible to correctthe third order spherical aberration that occurs due to the deviation inthe thickness of the light transmitting layer from the design lighttransmitting layer thickness. Meanwhile, when collecting light on theinformation recording surface L2 in which the thickness of the lighttransmitting layer is 69 μm and the information recording surface L3 inwhich the thickness of the light transmitting layer is 55 μm, converginglight is caused to enter the objective lens 8 by moving the collimatorlens 4 to the objective lens side. Consequently, it is possible tocorrect the third order spherical aberration that occurs due to thedeviation in the thickness of the light transmitting layer from thedesign light transmitting layer thickness.

Nevertheless, the variation in the light transmitting layer thickness ofthe information recording surfaces L0 to L3 is, for example, ±5 μm=50mλ, the initial aberration of the optical element such as the objectivelens is, for example, ±20 mλ, and the third order spherical aberrationthat occurs due to change in temperature or change in wavelength of thelight source is, for example, ±20 mλ. When giving consideration to thevariation in the third order spherical aberration, since the third orderspherical aberration 10 mλ, in NA=0.855 corresponds to a 1 μm thicknessof the light transmitting layer, it is necessary to give considerationto the variation in the third order spherical aberration correspondingto a ±9 μm thickness of the light transmitting layer upon recording orreproducing information to or from the respective information recordingsurfaces L0 to L3.

Accordingly, the thickness of the light transmitting layer of theinformation recording surface L0 will be 100±9 μm (91 to 109 μm), thethickness of the light transmitting layer of the information recordingsurface L1 will be 83±9 μm (74 to 92 μm), the thickness of the lighttransmitting layer of the information recording surface L2 will be 69±9μm (60 to 78 μm), and the thickness of the light transmitting layer ofthe information recording surface L3 will be 55±9 μm (46 to 64 μm).

As described above, the required moving range of the collimator lens 4in a predetermined information recording surface will overlap with therequired moving range of the collimator lens 4 of an adjacentinformation recording surface. Here, if the amount of the third orderspherical aberration corresponds to 80 μm which is equivalent to thedesign light transmitting layer thickness of the objective lens 8, thelaser beam that is emitted from the collimator lens 4 will be parallellight. Accordingly, with the optical head 30 of the first embodiment,the laser beam that is emitted from the collimator lens 4 may becomeparallel light upon recording or reproducing information to or from theinformation recording surface L1.

The mirror with a diffraction grating in the first embodiment is nowexplained.

As shown in FIG. 1, the mirror with a diffraction grating 5 of the firstembodiment mainly reflects the blue-violet laser beam that is emittedfrom the collimator lens 4, and bends the reflected part of theblue-violet laser beam in the direction of the objective lens 8.Moreover, the mirror with a diffraction grating 5 transmits the otherpart of the blue-violet laser beam, and causes the other transmittedpart of the blue-violet laser beam to enter the front monitor sensor 24.

The mirror with a diffraction grating 5 includes a first surface 5 awhich the blue-violet laser beam that was emitted from the blue-violetlaser light source 1 enters, and a second surface 5 b facing the firstsurface 5 a. The first surface 5 a and the second surface 5 b aremutually parallel. A reflective coat for transmitting and reflecting theblue-violet laser beam at a predetermined ratio is formed on the firstsurface 5 a, and a diffraction grating is formed on the second surface 5b.

FIG. 6 is a diagram showing the schematic configuration of the mirrorwith a diffraction grating 5 in the first embodiment of the presentinvention. The mirror with a diffraction grating 5 is a planar mirror. Areflective coat that reflects 90% reflects and 10% transmits theblue-violet laser beam that enters at an angle of 45 deg is formed onthe surface of the light incident side (first surface) 5 a of the mirrorwith a diffraction grating 5. Meanwhile, a diffraction grating parallelto the incident plane of the blue-violet laser beam that enters at anangle of 45 deg is formed on the surface of the light emission side(second surface) 5 b of the mirror with a diffraction grating 5.

The diffraction grating has a concave and convex pattern in which aplurality of straight lines parallel to the incident plane of theblue-violet laser beam are aligned in parallel. The diffraction gratingsplits the blue-violet laser beam that passed through the second surface5 b into zero order light and ±first order diffracted light, and splitsthe blue-violet laser beam that was internally reflected by the secondsurface 5 b into zero order light and ±first order diffracted light. Ageneral AR (Anti-Reflection) coat is formed on the second surface 5 bfor inhibiting the internal reflection. The first surface 5 a and thesecond surface 5 b are mutually parallel.

This kind of mirror with a diffraction grating 5 can be prepared bycutting a parallel plane substrate in which the foregoing reflectivecoat is formed on the first surface 5 a and the foregoing diffractiongrating and AR coat are formed on the second surface 5 b. Accordingly,it is possible to create this inexpensively in comparison to awedge-shaped reflective mirror, a wedge-shaped beam splitter or thelike.

Here, since the blue-violet laser beam that passes through the secondsurface 5 b on which a diffraction grating is formed and the blue-violetlaser beam that is reflected by the second surface 5 b will have adifferent phase difference caused by the depth d of the diffractiongrating, the diffraction efficiency of the zero order light and ±firstorder diffracted light is respectively different.

FIG. 7 is a diagram showing the condition of the transmitted light andthe reflected light of the mirror with a diffraction grating in thefirst embodiment of the present invention. As shown in FIG. 7, theblue-violet laser beam that entered the first surface 5 a of the mirrorwith a diffraction grating 5 at an angle θ1 (=45 deg) is refracted bythe first surface 5 a, and enters the second surface 5 b at an angle θ2(=28.75 deg). If the depth of the diffraction grating is d and therefractive index of the diffraction grating is n, the phase differenceδ1 that occurs upon passing through the diffraction grating will be thedifference between d×(1/cos(θ1)) and d×(n/cos(θ2)) as shown in followingFormula (1).δ1=d×{1/cos(θ1)−n/cos(θ2)}  (1)

Meanwhile, the phase difference δ2 that occurs upon the internalreflection of the diffraction grating will be as following Formula (2).δ2=2d×n×cos(θ2)  (2)

FIG. 8 shows the relationship between the depth d of the diffractiongrating, and the respective diffraction efficiencies of the transmittedzero order light that passed through the second surface 5 b, the ±firstorder diffracted light that passed through the second surface 5 b, thezero order light that was internally reflected by the second surface 5b, and the ±first order diffracted light that was internally reflectedby the second surface 5 b. The calculation conditions of the diffractiongrating are as follows. Specifically, the design wavelength λ of thediffraction grating is 405 nm, the angle of incidence θ1 of theblue-violet laser beam to the first surface is 45 deg, and therefractive index n of the diffraction grating is 1.47.

In FIG. 8, the horizontal axis shows the depth d of the diffractiongrating, and the vertical axis shows the diffraction efficiencies of thetransmitted zero order light, the transmitted±first order diffractedlight, the internally reflected zero order light and the internallyreflected±first order diffracted light. In addition, in FIG. 8, thewhite circles show the zero order light that passed through the secondsurface 5 b, the white squares show the ±first order diffracted lightthat passed through the second surface 5 b, the black triangles show thezero order light that was internally reflected by the second surface 5b, and the black squares show the ±first order diffracted light that wasinternally reflected by the second surface 5 b.

As shown in FIG. 8, when the depth d of the diffraction grating is 0.08μm, the diffraction efficiency (reflectance) of the internally reflectedzero order light will be approximately zero, and the diffractionefficiency of the internally reflected±first order diffracted light willbe 41%, respectively. Here, the diffraction efficiency (transmittance)of the transmitted zero order light will be 97%, and the diffractionefficiency of the transmitted±first order diffracted light will be 1%.

As described above, in the first embodiment, as a result of using thedifference between the phase difference δ1 and the phase difference δ2when the depth d of the diffraction grating formed on the second surfaceof the mirror with a diffraction grating 5 is set to a predeterminedvalue, the diffraction grating is designed so as to satisfy followingFormula (3) and Formula (4).Diffraction efficiency of transmitted zero order light>diffractionefficiency of transmitted±first order diffracted light  (3)Diffraction efficiency of internally reflected zero orderlight<diffraction efficiency of internally reflected±first orderdiffracted light  (4)

In the mirror with a diffraction grating 5 of the first embodiment, thedepth d of the diffraction grating that is formed on the second surface5 b is 0.08 μm. Thus, the diffraction efficiency (reflectance) of thezero order light of the blue-violet laser beam that is headed toward thefront monitor sensor 24 after being internally reflected at least two ormore times by the mirror with a diffraction grating 5 will besubstantially zero. Moreover, the ±first order diffracted light of theblue-violet laser beam that is headed toward the front monitor sensor 24after being internally reflected at least two or more times by themirror with a diffraction grating 5 will be diffracted by thediffraction grating parallel to the incident plane of the laser beamthat enters at an angle of 45 deg. Accordingly, the blue-violet laserbeam that is headed toward the front monitor sensor 24 after beinginternally reflected by the mirror with a diffraction grating 5 will notbe parallel to the blue-violet laser beam that is headed toward thefront monitor sensor 24 after passing through the mirror with adiffraction grating 5 even if the blue-violet laser beam that enters themirror with a diffraction grating 5 is parallel light.

As described above, with the optical head which detects an APC signal bytransmitting a part of the blue-violet laser beam that entered theplanar mirror with a diffraction grating 5 and causing it to enter thefront monitor sensor 24, as a result of using the mirror with adiffraction grating 5 of the first embodiment, it is possible to inhibitthe interference of the blue-violet laser beam in the effective regionof the front monitor sensor 24 even if the blue-violet laser beam thatenters the mirror with a diffraction grating 5 is parallel light, and itis thereby possible to obtain an APC signal that is accuratelyproportionate to the quantity of outgoing light of the blue-violet laserlight source 1.

Accordingly, an inexpensive planar reflective mirror can also be used inan optical head for a multilayer optical disc in which the space betweenthe adjacent information recording surfaces is small and there is nochoice but to record or reproduce information at a position where thelaser beam that is emitted from the collimator lens becomes parallellight in the movable range of the collimator lens.

Moreover, the blue-violet laser beam that entered the mirror with adiffraction grating 5 passes through the second surface 5 b, isinternally reflected by the second surface 5 b and the first surface 5a, and subsequently passes through the second surface 5 b. Here, theblue-violet laser beam that passed through the second surface 5 bwithout being internally reflected and the blue-violet laser beam thatpassed through the second surface 5 b after being internally reflectedwill be diffracted with respectively different diffraction efficienciesbased on the diffraction grating.

Accordingly, even if the blue-violet laser beam that enters the mirrorwith a diffraction grating 5 is parallel light, since the ±first orderdiffracted light of the blue-violet laser beam that passes through thesecond surface 5 b after being internally reflected by the mirror with adiffraction grating 5 will not be parallel to the blue-violet laser beamthat passed through the second surface 5 b without being internallyreflected by the mirror with a diffraction grating 5, it is possible toinhibit the interference of the blue-violet laser beam in the effectiveregion of the front monitor sensor 24 for controlling the laser power ofthe blue-violet laser light source 1, and it is thereby possible toaccurately control the laser power of the blue-violet laser beam that isemitted from the blue-violet laser light source 1.

Incidentally, since there is no need to enlarge the diffraction angle ofthe ±first order diffracted light in order to inhibit the interference,the pitch p of the diffraction grating may be sufficiently larger thanthe design wavelength λ of the laser beam; for example, it may beapproximately 10 to 100 μm. In other words, the pitch p of thediffraction grating satisfies p>λ.

Moreover, based on foregoing Formula (1) and Formula (2), if thediffraction efficiency (reflectance) of the internally reflected zeroorder light is approximately zero, the larger the angle of incidence θ1,the larger the diffraction efficiency (transmittance) of the transmittedzero order light. Nevertheless, the mirror with a diffraction grating 5that is applied to the optical head of the first embodiment uses areflective mirror for reflecting the laser beam that is emitted from thecollimator lens so that it enters the information recording surface ofthe multilayer optical disc perpendicularly. In the foregoing case, theangle of incidence θ1 is preferably 45±10 [deg], and the angle ofincidence θ1 is more preferably 45±3 [deg].

Meanwhile, as shown in FIG. 8, as a result of the depth d of thediffraction grating deviating from 0.08 μm, the diffraction efficiency(reflectance) of the internally reflected zero order light will deviatefrom the design value (=0). For example, if the depth d of thediffraction grating is in the range of 0.08±0.01 μm, the diffractionefficiency (reflectance) of the internally reflected zero order lightwill be less than 5%. As described above, an anti-reflection AR coat isformed on the second surface 5 b of the mirror with a diffractiongrating 5, and the inner reflectance is suppressed to be less than 2%.Accordingly, since the diffraction efficiency (reflectance) of theinternally reflected zero order light will be less than 0.1%, it ispossible to sufficiently inhibit the interference of the laser beam inthe effective region of the front monitor sensor 24.

Specifically, if a glass material having a standard refractive index n(1.3≦n≦1.7) is used, it is possible to inhibit the diffractionefficiency (reflectance) of the substantially internally reflected zeroorder light to a problem-free level by the depth d of the diffractiongrating satisfying λ/6≦d≦λ/4 in relation to the design wavelength λ (forexample, λ=405 nm).

Although the mirror with a diffraction grating 5 of the first embodimentwas explained based on a case of forming a diffraction grating parallelto the incident plane of the blue-violet laser beam that enters at anangle of 45 deg, the diffraction grating is not limited to the foregoingshape. Specifically, the gist of the present invention is to achieve ashape so that the internally reflected±first order light will not becomeparallel with the laser beam that is headed toward the front monitorsensor upon passing through the mirror with a diffraction grating and,for example, it is also possible to form a diffraction grating that isperpendicular to the incident plane of the blue-violet laser beam thatenters at an angle of 45 deg, or form an annular diffraction grating.Moreover, the cross section shape is not limited to the binarydiffraction grating as shown in FIG. 6, and it may also be a blazeddiffraction grating. Since a binary diffraction grating can be easilyformed by way of etching or the like, it is more preferable as themirror with a diffraction grating of this embodiment.

FIG. 9 is a diagram showing the schematic configuration of the mirrorwith a diffraction grating in a modified example of the first embodimentof the present invention. The mirror with a diffraction grating 5′ shownin FIG. 9 is a planar mirror. A reflective coat that reflects 90%reflects and 10% transmits the blue-violet laser beam that enters at anangle of 45 deg is formed on the surface of the light incident side(first surface) 5 a′ of the mirror with a diffraction grating 5′.Meanwhile, a diffraction grating having an annular concave and convexpattern around the incident light axis of the blue-violet laser beam isformed on the surface of the light emission side (second surface) 5 b′of the mirror with a diffraction grating 5′.

The diffraction grating splits the blue-violet laser beam that passedthrough the second surface 5 b′ into zero order light and ±first orderdiffracted light, and splits the blue-violet laser beam that wasinternally reflected by the second surface 5 b into zero order light and±first order diffracted light. A general AR (Anti-Reflection) coat isformed on the second surface 5 b′ for inhibiting the internalreflection. The first surface 5 a′ and the second surface 5 b′ aremutually parallel.

This kind of mirror with a diffraction grating 5′ can be prepared bycutting a parallel plane substrate in which the foregoing reflectivecoat is formed on the first surface 5 a′ and the foregoing diffractiongrating and AR coat are formed on the second surface 5 b′. Accordingly,it is possible to create this inexpensively in comparison to awedge-shaped reflective mirror, a wedge-shaped beam splitter or thelike.

Incidentally, since the mirror with a diffraction grating 5′ has thesame functions and characteristics as the mirror with a diffractiongrating 5 other than the pattern of the diffraction grating, thedetailed explanation thereof is omitted.

Preferably, the mirror with a diffraction grating 5 is formed in asuitable shape so that the internally reflected±first order light willnot reflect on the optical surface of the peripheral element or a planarsurface in the optical head and infiltrate the light receiving elementor the like; that is, so that the internally reflected±first order lightwill not become stray light.

With a multilayer optical disc in which the space between theinformation recording surfaces relatively large, the required movingrange of the collimator lens upon collecting the laser beam on apredetermined information recording surface and the required movingrange of the collimator lens upon collecting the laser beam on theinformation recording surface that is adjacent to such predeterminedinformation recording surface will not overlap. In the foregoing case,it is preferable to configure the optical head so that the position ofthe collimator lens in which the laser beam that is emitted from thecollimator lens will become parallel light will be outside the requiredmoving range of the collimator lens upon collecting the laser beam onany of the information recording surfaces.

As a result of adopting the foregoing configuration, since the laserbeam that is emitted from the collimator lens will become converginglight or diverging light in the required moving range of the collimatorlens upon collecting the laser beam on any one of the informationrecording surfaces, it is possible to inhibit the interference of thelaser beam in the effective region of the front monitor sensor even whenusing a planar beam splitter that is not equipped with a diffractiongrating.

Second Embodiment

FIG. 10 shows the schematic configuration of the optical head in thesecond embodiment of the present invention. In the second embodiment,the same reference numeral is given to the same constituent element thatis common with the first embodiment, and the explanation thereof isomitted.

An optical head 40 shown in FIG. 10 comprises a blue-violet laser lightsource 1, a relay lens 2, a polarizing beam splitter 3, a collimatorlens 4, a mirror with a diffraction grating 25, a quarter wavelengthplate 6, a diffraction lens 7, an objective lens 8, an objective lensactuator 9, a dual wavelength laser light source 11, a diffractiongrating 12, a planar beam splitter 13, a collimator lens actuator 14, aplanar mirror 15, a quarter wavelength plate 16, a compatible objectivelens 18, a detection hologram 21, a detection lens 22, a light receivingelement 23 and a front monitor sensor 24.

Moreover, as shown in FIG. 2, the multilayer optical disc 60 has fourinformation recording surfaces L0 to L3 each having a light transmittinglayer with a thickness d0 to d3 of 55 μm, 69 μm, 83 μm, and 100 μmrespectively.

The dual wavelength laser light source 11 emits a red laser beam havinga second wavelength λ2 (for example, approximately 660 nm) that islarger than the first wavelength λ1 (for example, approximately 405 nm),and emits an infrared laser beam having a third wavelength λ3 (forexample, approximately 785 nm) that is larger than the second wavelengthλ2. The planar mirror 15 transmits and reflects the red laser beam at apredetermined ratio, and substantially transmits the blue-violet laserbeam. The planar mirror 15 transmits and reflects the infrared laserbeam at a predetermined ratio.

The compatible objective lens 18 converges the red laser beam on aninformation recording surface of a DVD 70 of a type that is differentfrom the multilayer optical disc 60. The compatible objective lens 18converges the infrared laser beam on an information recording surface ofa CD 80 of a type that is different from the multilayer optical disc 60and the DVD 70.

The collimator lens actuator 14 causes diverging light or converginglight to enter the compatible objective lens 18 upon recording orreproducing information to or from the information recording surface ofthe DVD 70 or the CD 80. More specifically, the collimator lens actuator14 causes converging light to enter the compatible objective lens 18upon recording or reproducing information to or from the informationrecording surface of the DVD 70, and causes diverging light to enter thecompatible objective lens 18 upon recording or reproducing informationto or from the information recording surface of the CD 80.

The operation of the optical head 40 upon recording or reproducinginformation to or from the multilayer optical disc 60 is now explained.The blue-violet laser beam having a wavelength of approximately 405 nmthat was emitted from the blue-violet laser light source 1 is convertedinto diverging light having a different NA by the relay lens 2, andenters the polarizing beam splitter 3 by S polarization. The laser beamthat was reflected by the polarizing beam splitter 3 is converted intoapproximately parallel light with the collimator lens 4, passes throughthe planar mirror 15, and enters the mirror with a diffraction grating25. A part of the blue-violet laser beam that entered the mirror with adiffraction grating 25 is reflected in the direction of the quarterwavelength plate 6. The other part of the laser beam that entered themirror with a diffraction grating 25 passes through the mirror with adiffraction grating 25, and subsequently enters the front monitor sensor24. Then the output of the blue-violet laser light source 1 iscontrolled based on the output of the front monitor sensor 24.

Meanwhile, the laser beam that was reflected by the mirror with adiffraction grating 25 is converted into circular polarized light by thequarter wavelength plate 6, and thereafter passes through thediffraction lens 7. The laser beam that passed through the diffractionlens 7 is converged as a light spot onto any one of the informationrecording surfaces L0 to L3 of the multilayer optical disc 60 by theobjective lens 8.

The blue-violet laser beam that is reflected by the predeterminedinformation recording surface of the multilayer optical disc 60 passesthrough the objective lens 8 and the diffraction lens 7 once again, isconverted into linear polarized light that is different from the outwardpath by the quarter wavelength plate 6, and subsequently reflected bythe mirror with a diffraction grating 25. The laser beam that isreflected by the mirror with a diffraction grating 25 passes through theplanar mirror 15 and the collimator lens 4, and subsequently enters thepolarizing beam splitter 3 by P polarization. The laser beam that passedthrough the polarizing beam splitter 3 is guided to the light receivingelement 23 via the detection hologram 21 and the detection lens 22. Thelaser beam that was detected with the light receiving element 23 issubject to photoelectric conversion. The signal created by photoelectricconversion is operated with the control unit described later, and afocus error signal for following the surface fluctuation of themultilayer optical disc 60 and a tracking error signal for following theeccentricity of the multilayer optical disc 60 are created.

The operation of the optical head 40 upon recording or reproducinginformation to or from the DVD 70 is now explained. The red laser beamhaving a wavelength of approximately 660 nm that was emitted from thedual wavelength laser light source 11 is split into a main beam as thezero order light and a sub beam as the ±first order diffracted light bythe diffraction grating 12. The main beam and the sub beam are reflectedby the planar beam splitter 13, and pass through the polarizing beamsplitter 3. The red laser beam that passed through the polarizing beamsplitter 3 is converted into converging light with the collimator lens4, and enters the planar mirror 15. A part of the red laser beam thatentered the planar mirror 15 is reflected in the direction of thequarter wavelength plate 16. The other part of the red laser beam thatentered the planar mirror 15 passes through the planar mirror 15 and themirror with a diffraction grating 25, and subsequently enters the frontmonitor sensor 24. Then, the output of the red laser beam of the dualwavelength laser light source 11 is controlled based on the output ofthe front monitor sensor 24.

Meanwhile, the red laser beam that was reflected by the planar mirror 15is converted into circular polarized light with the quarter wavelengthplate 16, and subsequently converged as a light spot on the informationrecording surface of the DVD 70 by the compatible objective lens 18.

The red laser beam that was reflected by the information recordingsurface of the DVD 70 pass through the compatible objective lens 18 onceagain, is converted into linear polarized light that is different fromthe outward path by the quarter wavelength plate 16, and subsequentlyreflected by the planar mirror 15. The red laser beam that was reflectedby the planar mirror 15 passed through the collimator lens 4, andsubsequently enters the polarizing beam splitter 3 and the planar beamsplitter 13 by P polarization. The red laser beam that passed throughthe polarizing beam splitter 3 and the planar beam splitter 13 is guidedto the light receiving element 23 via the detection hologram 21 and thedetection lens 22. The red laser beam that is detected with the lightreceiving element 23 is subject to photoelectric conversion. The signalcreated by photoelectric conversion is operated with the control unitdescribed later, and a focus error signal for following the surfacefluctuation of the DVD 70 and a tracking error signal for following theeccentricity of the DVD 70 are created.

The operation of the optical head 40 upon recording or reproducinginformation to or from the CD 80 is explained. The infrared laser beamhaving a wavelength of approximately 785 nm that was emitted from thedual wavelength laser light source 11 is split into a main beam as thezero order light and a sub beam as a ±first order diffracted light bythe diffraction grating 12. The main beam and the sub beam are reflectedby the planar beam splitter 13, and enter the polarizing beam splitter3. The infrared laser beam that passed through the polarizing beamsplitter 3 is converted into diverging light with a different NA by thecollimator lens 4, and enters the planar mirror 15. A part of theinfrared laser beam that entered the planar mirror 15 is reflected inthe direction of the quarter wavelength plate 16. The other part of theinfrared laser beam that entered the planar mirror 15 passes through theplanar mirror 15 and the mirror with a diffraction grating 25, andsubsequently enters the front monitor sensor 24. Then, the output of theinfrared laser beam of the dual wavelength laser light source 11 iscontrolled based on the output of the front monitor sensor 24.

Meanwhile, the infrared laser beam that was reflected by the planarmirror 15 is converted into circular polarized light by the quarterwavelength plate 16, and subsequently converged as a light spot on theinformation recording surface of the CD 80 by the compatible objectivelens 18.

The infrared laser beam that was reflected by the information recordingsurface of the CD 80 passes through the compatible objective lens 18once again, is converted into linear polarized light that is differentfrom the outward path by the quarter wavelength plate 16, andsubsequently reflected by the planar mirror 15. The infrared laser beamthat was reflected by the planar mirror 15 passes through the collimatorlens 4, and subsequently enters the polarizing beam splitter 3 and theplanar beam splitter 13 by P polarization. The infrared laser beam thatpassed through the polarizing beam splitter 3 and the planar beamsplitter 13 is guided to the light receiving element 23 via thedetection hologram 21 and the detection lens 22. The infrared laser beamthat was detected with the light receiving element 23 is subject tophotoelectric conversion. The signal that was created by photoelectricconversion is operated with the control unit described later, and afocus error signal for following the surface fluctuation of the CD 80and a tracking error signal for following the eccentricity of the CD 80are created.

Incidentally, in the second embodiment, the blue-violet laser beamcorresponds to an example of the first laser beam, the blue-violet laserlight source 1 corresponds to an example of the first light source, themirror with a diffraction grating 25 corresponds to an example of thefirst planar optical element, the multilayer optical disc 60 correspondsto an example of the first information recording medium, the objectivelens 8 corresponds to an example of the first objective lens, the lightreceiving element 23 corresponds to an example of the firstphotodetector, the front monitor sensor 24 corresponds to an example ofthe second photodetector, the collimator lens actuator 14 corresponds toan example of the spherical aberration correction unit, the red laserbeam corresponds to an example of the second laser beam, the infraredlaser beam corresponds to an example of the third laser beam, the dualwavelength laser light source 11 corresponds to an example of the secondlight source and the third light source, the planar mirror 15corresponds to an example of the second planar optical element, the DVD70 corresponds to an example of the second information recording medium,the compatible objective lens 18 corresponds to an example of the secondobjective lens, and the CD 80 corresponds to an example of the thirdinformation recording medium.

The detection of the focus error signal and the detection of thetracking error signal in the optical head of the second embodiment arenow explained.

The focus error signal for following the surface fluctuation of the DVD70 and the CD 80 is detected using the so-called astigmatism method orthe like of detecting the focal spot provided by the detection lens 22based on astigmatism with a quartered light receiving pattern in thelight receiving element 23.

Meanwhile, the tracking error signal for following the eccentricity ofthe DVD 70 and the CD 80 is detected using the so-called three beammethod or differential push-pull method (DPP method) using the main beamand the sub beam which are created with the diffraction grating 12.

The compatible objective lens in the second embodiment is now explained.

The compatible objective lens 18 comprises a diffraction structure forrespectively collecting the red laser beam for recording or reproducinginformation to or from the DVD 70 and the infrared laser beam forrecording or reproducing information to or from the CD 80 as a finelight spot by using the difference in wavelength.

In the second embodiment, when recording or reproducing information toor from the DVD 70, converging light is caused to enter the compatibleobjective lens 18 at a predetermined convergent angle by moving thecollimator lens 4 to the objective lens side. Moreover, when recordingor reproducing information to or from the CD 80, diverging light iscaused to enter the compatible objective lens 18 at a predetermineddivergence angle by moving the collimator lens 4 to the light sourceside. The compatible objective lens 18 of the second embodiment isdesigned so that the third order spherical aberration will be minimal inrelation to the thickness of the light transmitting layer of the DVD 70or the CD 80.

Incidentally, as shown in FIG. 5A, when the collimator lens 4 is in thereference position, the outgoing beam of the collimator lens 4 will beapproximately parallel light. Meanwhile, as shown in FIG. 5B, as aresult of moving the collimator lens 4 from the reference position to apredetermined position on the light source side, the outgoing beam ofthe collimator lens 4 becomes diverging light having a predetermineddivergence angle. It is thereby possible to record or reproduceinformation to or from the CD 80. Meanwhile, as shown in FIG. 5C, as aresult of moving the collimator lens 4 from the reference position to apredetermined position on the objective lens side, the outgoing beam ofthe collimator lens 4 becomes converging light having a predeterminedconvergent angle. It is thereby possible to record or reproduceinformation to or from the DVD 70.

As with the first embodiment, in the multilayer optical disc 60 having aplurality of information recording surfaces, it is possible to correctthe third order spherical aberration by moving the collimator lens 4according to the thickness of the light transmitting layer of therespective information recording surfaces.

Incidentally, the compatible objective lens 18 is not limited to anobjective lens comprising the foregoing diffraction structure, and itmay also be a refraction-type objective lens that uses the wavelengthdispersion characteristics of a plurality of glass materials, or a pairlens that combines a plurality of diffraction-type and refraction-typelenses.

As described above, the optical head 40 of the second embodimentcomprises compatibility, and it is possible to record or reproduceinformation to or from different types of optical discs; for instance,to or from the multilayer optical disc 60, the DVD 70 and the CD 80.

The mirror with a diffraction grating in the second embodiment is nowexplained.

As shown in FIG. 10, the mirror with a diffraction grating 25 of thesecond embodiment mainly reflects the blue-violet laser beam that isemitted from the collimator lens 4, and bends most of the reflected partof the blue-violet laser beam in the direction of the objective lens 8.Moreover, the mirror with a diffraction grating 25 transmits a part ofthe blue-violet laser beam, most of the red laser beam, and most of theinfrared laser beam, and causes the respective transmitted laser beamsto enter the front monitor sensor 24.

The mirror with a diffraction grating 25 is a planar mirror. Areflective coat having wavelength selectivity that reflects 90% reflectsand 10% transmits the blue-violet laser beam that enters at an angle of45 deg and reflects nearly 100% of the red laser beam and the infraredlaser beam is formed on the surface of the light incident side (firstsurface) 25 a of the mirror with a diffraction grating 25.

Meanwhile, a diffraction grating parallel to the entrance place of theblue-violet laser beam that enters at an angle of 45 deg is formed onthe surface of the light emission side (second surface) 25 b of themirror with a diffraction grating 25. The diffraction grating splits theblue-violet laser beam that passed through the second surface 25 b intozero order light and ±first order diffracted light, and splits theblue-violet laser beam that was internally reflected by the secondsurface 25 b into zero order light and ±first order diffracted light. AnAR coat corresponding to the three wavelengths of the blue-violet laserbeam, the red laser beam and the infrared laser beam is formed on thesecond surface 25 b for inhibiting the internal reflection. The firstsurface 25 a and the second surface 25 b are mutually parallel.

This kind of mirror with a diffraction grating 25 can be prepared bycutting a parallel plane substrate in which the foregoing reflectivecoat having wavelength selectivity is formed on the first surface 25 aand the foregoing diffraction grating and AR coat are formed on thesecond surface 25 b. Accordingly, it is possible to create thisinexpensively in comparison to a wedge-shaped reflective mirror, awedge-shaped beam splitter or the like.

In the mirror with a diffraction grating 25 of the second embodiment, aswith the mirror with a diffraction grating 5 explained in the firstembodiment, the depth d of the diffraction grating that is formed on thesecond surface 25 b is 0.08 μm. Thus, the diffraction efficiency(reflectance) of the zero order light of the blue-violet laser beam thatis headed toward the front monitor sensor 24 after being internallyreflected at least two or more times by the mirror with a diffractiongrating 25 will be substantially zero. Moreover, the ±first orderdiffracted light of the blue-violet laser beam that is headed toward thefront monitor sensor 24 after being internally reflected at least two ormore times by the mirror with a diffraction grating 25 will bediffracted by the diffraction grating parallel to the incident plane ofthe laser beam that enters at an angle of 45 deg. Accordingly, theblue-violet laser beam that is headed toward the front monitor sensor 24after being internally reflected by the mirror with a diffractiongrating 25 will not be parallel to the blue-violet laser beam that isheaded toward the front monitor sensor 24 after passing through themirror with a diffraction grating 25 even if the blue-violet laser beamthat enters the mirror with a diffraction grating 25 is parallel light.

Meanwhile, the diffraction efficiency (transmittance) of the zero orderlight of the red laser beam and the infrared laser beam that is headedtoward the front monitor sensor 24 after passing through the secondsurface 25 b will be 98% or higher. The zero order light transmittanceη1 of the blue-violet laser beam in the diffraction grating is smallerthan the zero order light transmittance η2 of the red laser beam and theinfrared laser beam in the diffraction grating. Moreover, thereflectance of the red laser beam and the infrared laser beam in thefirst surface 25 a is sufficiently small and is less than 5%. Thus, thequantity of light of the red laser beam and the infrared laser beam thatis headed toward the front monitor sensor 24 after being internallyreflected at least two or more times by the mirror with a diffractiongrating 25 will become sufficiently small.

The red laser beam and the infrared laser beam respectively enter theplanar mirror 15 as converging light and diverging light. Specifically,the collimator lens actuator 14 causes converging light to enter thecompatible objective lens 18 upon recording or reproducing informationto or from the information recording surface of the DVD 70, and causesdiverging light to enter the compatible objective lens 18 upon recordingor reproducing information to or from the information recording surfaceof the CD 80. Thus, the optical axis of the laser beam that passedthrough the planar mirror 15 and the mirror with a diffraction grating25 and is headed toward the front monitor sensor 24 and the optical axisof the laser beam that was internally reflected two or more times in theplanar mirror 15, subsequently entered the mirror with a diffractiongrating 25, passed through the mirror with a diffraction grating 25, andis subsequently headed toward the front monitor sensor 24 will not bemutually parallel.

As described above, with the optical head which detects an APC signal bytransmitting a part of the blue-violet laser beam that entered theplanar mirror with a diffraction grating 25 and causing it to enter thefront monitor sensor 24, as a result of using the mirror with adiffraction grating 25 of the second embodiment, it is possible toinhibit the interference of the laser beam in the effective region ofthe front monitor sensor 24 even if the laser beam that enters themirror with a diffraction grating 25 is parallel light, and it isthereby possible to obtain an APC signal that is accuratelyproportionate to the quantity of outgoing light of the blue-violet laserlight source 1 and the dual wavelength laser light source 11.

Incidentally, although an AR coat correspond to the three wavelengths ofthe blue-violet laser beam, the red laser beam and the infrared laserbeam is formed on the second surface 25 b of the mirror with adiffraction grating 25 in the second embodiment, the present inventionis not limited thereto. An AR coat having prescribed reflectance andtransmittance in relation to the blue-violet laser beam may be formed onthe second surface 25 b of the mirror with a diffraction grating 25 inorder to prevent the internal reflection of the blue-violet laser beam.Since this AR coat is optimized for the blue-violet laser beam, several% of the red laser beam and the infrared laser beam will be internallyreflected. Specifically, the reflectance for the blue-violet laser beamis less than 1% and the reflectance for the red laser beam and theinfrared laser beam is 2% or more.

In other words, the reflectance R1 for the blue-violet laser beam in theAR coat (antireflection coat) and the reflectance R2 for the red laserbeam and the infrared laser beam in the AR coat satisfy R1<(R2)/2.

Moreover, in the second embodiment, although the blue-violet laser lightsource 1 for emitting the blue-violet laser beam and the dual wavelengthlaser light source 11 for emitting the red laser beam and the infraredlaser beam are provided, the present invention is not limited thereto,and it is also possible to provide the blue-violet laser light source 1for emitting the blue-violet laser beam and a laser light source foremitting either the red laser beam or the infrared laser beam.

Third Embodiment

FIG. 11 is a diagram showing the schematic configuration of the opticalhead in the third embodiment of the present invention. In the thirdembodiment, the same reference numeral is given to the same constituentelement that is common with the first embodiment and the secondembodiment, and the explanation thereof is omitted.

The optical head 41 shown in FIG. 11 comprises a wavelength selectionmirror 35 in substitute for the mirror with a diffraction grating 25 ofthe optical head 40 shown in FIG. 10. The wavelength selection mirror 35mainly reflects the blue-violet laser beam that is emitted from thecollimator lens 4, and bends most of the reflected part of theblue-violet laser beam in the direction of the objective lens 8.Moreover, the wavelength selection mirror 35 transmits a part of theblue-violet laser beam, most of the red laser beam, and most of theinfrared laser beam, and causes the respective transmitted laser beamsto enter the front monitor sensor 24. Incidentally, in the thirdembodiment, the wavelength selection mirror 35 corresponds to an exampleof the first planar optical element.

The wavelength selection mirror 35 is a planar mirror having wavelengthselectivity. A reflective coat having wavelength selectivity thatreflects 90% reflects and 10% transmits the blue-violet laser beam thatenters at an angle of 45 deg and reflects nearly 100% of the red laserbeam and the infrared laser beam is formed on the surface of the lightincident side (first surface) 35 a of the wavelength selection mirror35.

Meanwhile, an AR coat having prescribed reflectance and transmittancefor the blue-violet laser beam is formed on the surface of the lightemission side (second surface) 35 b of the wavelength selection mirror35 in order to prevent the internal reflection of the blue-violet laserbeam. Since this AR coat is optimized for the blue-violet laser beam,several % of the red laser beam and the infrared laser beam will beinternally reflected. Specifically, the reflectance for the blue-violetlaser beam is less than 1% and the reflectance for the red laser beamand the infrared laser beam is 2% or more. Incidentally, the firstsurface 35 a and the second surface 35 b are mutually parallel.

This kind of wavelength selection mirror 35 can be prepared by cutting aparallel plane substrate in which the foregoing reflective coat havingwavelength selectivity is formed on the first surface 35 a and theforegoing AR coat is formed on the second surface 35 b. Accordingly, itis possible to create this extremely inexpensively in comparison to awedge-shaped reflective mirror, a wedge-shaped beam splitter or thelike.

With the wavelength selection mirror 35 of the third embodiment, thereflectance of the blue-violet laser beam in the second surface 35 b isless than 1%. Specifically, the reflectance R1 for the blue-violet laserbeam in the AR coat (antireflection coat) is smaller than 0.01. Thus,the quantity of light of the blue-violet laser beam that is headedtoward the front monitor sensor 24 after being internally reflected atleast two or more times by the wavelength selection mirror 35 will besufficiently small.

Meanwhile, although the reflectance of the red laser beam and theinfrared laser beam in the second surface 35 b is 2% or higher, thereflectance of the red laser beam and the infrared laser beam in thefirst surface 35 a is sufficiently small at less than 5%. Thus, thequantity of light of the red laser beam and the infrared laser beam thatis headed toward the front monitor sensor 24 after being internallyreflected at least two or more times by the wavelength selection mirror35 will be sufficiently small.

As described above, with the optical head which detects an APC signal bytransmitting a part of the blue-violet laser beam that entered theplanar wavelength selection mirror 35 and causing it to enter the frontmonitor sensor 24, as a result of using the wavelength selection mirror35 of the third embodiment, it is possible to inhibit the interferenceof the laser beam in the effective region of the front monitor sensor 24even if the laser beam that enters the wavelength selection mirror 35 isparallel light, and it is thereby possible to obtain an APC signal thatis accurately proportionate to the quantity of outgoing light of theblue-violet laser light source 1 and the dual wavelength laser lightsource 11.

Incidentally, as a result of applying the AR coat that is optimized forthe blue-violet laser beam explained in the third embodiment to themirror with a diffraction grating 5 and the mirror with a diffractiongrating 25 described in the first embodiment and the second embodiment,it is possible to further inhibit the interference of the laser beam inthe effective region of the front monitor sensor 24.

Fourth Embodiment

FIG. 12 is a diagram showing the schematic configuration of the opticalhead in the fourth embodiment of the present invention. In the fourthembodiment, the same reference numeral is given to the same constituentelement that is common with the first embodiment, and the explanationthereof is omitted.

In FIG. 12, an optical head 42 comprises a blue-violet laser lightsource 1 for emitting a blue-violet laser beam, a collimator lens 44, aplanar beam splitter with a diffraction grating 33, a reflective mirror45, a quarter wavelength plate 6, a diffraction lens 7, an objectivelens 8, an objective lens actuator 9, a detection lens 46, a lightreceiving element 23 and a front monitor sensor 34.

The operation of the optical head 42 upon recording or reproducinginformation to or from the multilayer optical disc 60 is now explained.The blue-violet laser beam having a wavelength of approximately 405 nmthat was emitted from the blue-violet laser light source 1 is convertedinto approximate parallel light by the collimator lens 44, and entersthe planar beam splitter with a diffraction grating 33 by Spolarization. A part of the blue-violet laser beam t planar beamsplitter with a diffraction grating 33 is reflected by the front monitorsensor 34 after passing through the planar beam splitter with adiffraction grating 33. Then the output of the blue-violet laser lightsource 1 is controlled based on the output of the front monitor sensor34.

Meanwhile, the other part of the blue-violet laser beam that wasreflected by the planar beam splitter with a diffraction grating 33 isreflected in the direction of the quarter wavelength plate 6 by thereflective mirror 45. The blue-violet laser beam that was reflected bythe reflective mirror 45 is converted into circular polarized light withthe quarter wavelength plate 6, and subsequently passes through thediffraction lens 7. The blue-violet laser beam that passed through thediffraction lens 7 is converged, as a light spot, on any of theinformation recording surfaces L0 to L3 of the multilayer optical disc60 by the objective lens 8.

The blue-violet laser beam that was reflected by the predeterminedinformation recording surface of the multilayer optical disc 60 passesthrough the objective lens 8 and the diffraction lens 7 once again, isconverted into linear polarized light that is different from the outwardpath by the quarter wavelength plate 6, and subsequently reflected bythe reflective mirror 45. The blue-violet laser beam that was reflectedby the reflective mirror 45 enters the planar beam splitter with adiffraction grating 33 by P polarization. The blue-violet laser beamthat passed through the planar beam splitter with a diffraction grating33 is guided to the light receiving element 23 via the detection lens46. The laser beam that was detected with the light receiving element 23is subject to photoelectric conversion. The signal created byphotoelectric conversion is operated with the control unit describedlater, and a focus error signal for following the surface fluctuation ofthe multilayer optical disc 60 and a tracking error signal for followingthe eccentricity of the multilayer optical disc 60 are created.

Incidentally, in the fourth embodiment, the blue-violet laser beamcorresponds to an example of the first laser beam, the blue-violet laserlight source 1 corresponds to an example of the first light source, theplanar beam splitter with a diffraction grating 33 corresponds to anexample of the first planar optical element, the multilayer optical disc60 corresponds to an example of the first information recording medium,the objective lens 8 corresponds to an example of the first objectivelens, the light receiving element 23 corresponds to an example of thefirst photodetector, and the front monitor sensor 34 corresponds to anexample of the second photodetector.

The planar beam splitter with a diffraction grating in the fourthembodiment is now explained.

As shown in FIG. 12, planar beam splitter with a diffraction grating 33of the fourth embodiment mainly reflects the blue-violet laser beam thatis emitted from the collimator lens 44, and bends most of the reflectedpart of the blue-violet laser beam in the direction of the reflectivemirror 45. Moreover, the planar beam splitter with a diffraction grating33 transmits a part of the blue-violet laser beam, and causes suchtransmitted part of the blue-violet laser beam to enter the frontmonitor sensor 34.

A polarizing coat that reflects 90% reflects and 10% transmits the Spolarization blue-violet laser beam that enters at an angle of 45 deg isformed on the surface of the light incident side (first surface) 33 a ofthe planar beam splitter with a diffraction grating 33.

Meanwhile, a diffraction grating parallel to the entrance place of theblue-violet laser beam that enters at an angle of 45 deg is formed onthe surface of the light emission side (second surface) 33 b of theplanar beam splitter with a diffraction grating 33. The diffractiongrating splits the blue-violet laser beam that passed through the secondsurface 33 b into zero order light and ±first order diffracted light,and splits the blue-violet laser beam that was internally reflected bythe second surface 33 b into zero order light and ±first orderdiffracted light. A general AR coat is formed on the second surface 33 bfor inhibiting the internal reflection. The first surface 33 a and thesecond surface 33 b are mutually parallel.

Incidentally, the diffraction grating that is formed on the secondsurface 33 b of the planar beam splitter with a diffraction grating 33in the fourth embodiment has the same functions and characteristics asthe diffraction grating that is formed on the second surface 5 b of themirror with a diffraction grating 5 in the first embodiment.

This kind of planar beam splitter with a diffraction grating 33 can beprepared by cutting a parallel plane substrate in which the foregoingpolarizing coat is formed on the first surface 33 a and the foregoingdiffraction grating and AR coat are formed on the second surface 33 b.Accordingly, it is possible to create this inexpensively in comparisonto a wedge-shaped reflective mirror, a wedge-shaped beam splitter or thelike.

In the planar beam splitter with a diffraction grating 33 of the fourthembodiment, as with the mirror with a diffraction grating 5 explained inthe first embodiment, the depth d of the diffraction grating that isformed on the second surface 33 b is 0.08 μm. Thus, the efficiency(reflectance) of the zero order light of the blue-violet laser beam thatis headed toward the front monitor sensor 34 after being internallyreflected at least two or more times by the planar beam splitter with adiffraction grating 33 will be substantially zero. Moreover, the ±firstorder diffracted light of the blue-violet laser beam that is headedtoward the front monitor sensor 34 after being internally reflected atleast two or more times by the planar beam splitter with a diffractiongrating 33 will be diffracted by the diffraction grating parallel to theincident plane of the laser beam that enters at an angle of 45 deg.Accordingly, the blue-violet laser beam that is headed toward the frontmonitor sensor 34 after being internally reflected by the planar beamsplitter with a diffraction grating 33 will not be parallel to theblue-violet laser beam that is headed toward the front monitor sensor 34after passing through the planar beam splitter with a diffractiongrating 33 even if the blue-violet laser beam that enters the planarbeam splitter with a diffraction grating 33 is parallel light.

As described above, with the optical head which detects an APC signal bytransmitting a part of the blue-violet laser beam that entered theplanar beam splitter with a diffraction grating 33 and causing it toenter the front monitor sensor 34, as a result of using the planar beamsplitter with a diffraction grating 33 of the fourth embodiment, it ispossible to inhibit the interference of the blue-violet laser beam inthe effective region of the front monitor sensor 34 even if theblue-violet laser beam that enters the planar beam splitter with adiffraction grating 33 is parallel light, and it is thereby possible toobtain an APC signal that is accurately proportionate to the quantity ofoutgoing light of the blue-violet laser light source 1.

Although the foregoing first embodiment to fourth embodiment explainedan optical head for recording or reproducing information to or from themultilayer optical disc 60 having four information recording surfaces L0to L3 each having a light transmitting layer with a thickness d0 to d3of 55 μm, 69 μm, 83 μm, and 100 μm respectively, the multilayer opticaldisc 60 is not limited to the foregoing structure. It goes withoutsaying that the optical head of the first to fourth embodiments can bebroadly applied to a multilayer optical disc having at least threeinformation recording surfaces each having a light transmitting layerdifferent in thickness; that is, a multilayer optical disc in which thespace between the adjacent information recording surfaces is small incomparison to the BD which has been put into practical use.

Fifth Embodiment

FIG. 13 is a diagram showing the schematic configuration of the opticaldisc device in the fifth embodiment of the present invention.

In FIG. 13, an optical disc device 50 internally comprises an opticaldisc drive unit 51, a control unit 52 and an optical head 53.

The optical disc drive unit 51 rotatively drives the multilayer opticaldisc 60 (or DVD 70 or CD 80). The optical head 53 is any one of theoptical heads explained in the first embodiment to the fourthembodiment. The control unit 52 controls the drive of the optical discdrive unit 51 and the optical head 53, and performs signal processing ofthe control signal and the information signal that were subject tophotoelectric conversion in the optical head 53. Moreover, the controlunit 52 has a function of interfacing the information signal with theoutside and inside of the optical disc device 50.

The control unit 52 receives the control signal obtained from theoptical head 53, and performs focus control, tracking control,information reproduction control, and rotative control of the opticaldisc drive unit 51 based on the control signal. The control unit 52reproduces information from the information signal and sends a recordingsignal to the optical head 53.

Since the optical disc device 50 is equipped with any one of the opticalheads explained in the first embodiment to the fourth embodiment, theoptical disc device 50 of the fifth embodiment is able to favorablerecord or reproduce information to or from a multilayer optical disccomprising at least three information recording surfaces.

Sixth Embodiment

FIG. 14 is a diagram showing the schematic configuration of the computerin the sixth embodiment of the present invention.

In FIG. 14, a computer 500 comprises the optical disc device 50 of thefifth embodiment, an input device 501 such as a keyboard, a mouse or atouch panel for inputting information, an arithmetic unit 502 such as acentral processing unit (CPU) for performing arithmetic operations basedon the information that was input from the input device 501 andinformation that was read from the optical disc device 50, and an outputdevice 503 such as a cathode-ray tube or liquid crystal display devicefor displaying the results of the arithmetic operation by the arithmeticunit 502 or a printer for printing information.

Incidentally, in the sixth embodiment, the computer 500 corresponds toan example of the information processing device, and the arithmetic unit502 corresponds to an example of the information processing unit.

Since the computer 500 comprises the optical disc device 50 of the fifthembodiment, it is possible to favorably record or reproduce informationto or from a multilayer optical disc comprising at least threeinformation recording surfaces, and can be applied to a variety of uses.

Seventh Embodiment

FIG. 15 is a diagram showing the schematic configuration of the opticaldisc player in the seventh embodiment of the present invention.

In FIG. 15, the optical disc player 600 comprises the optical discdevice 50 of the fifth embodiment, and a decoder 601 for converting theinformation signal obtained from the optical disc device 50 into animage signal.

Incidentally, the optical disc player 600 can also be used as a carnavigation system by adding a position sensor such as a GPS and acentral processing unit (CPU). The optical disc player 600 furthercomprises a display device 602 such as a liquid crystal monitor.

Incidentally, in the seventh embodiment, the optical disc player 600corresponds to an example of the information processing device, and thedecoder 601 corresponds to an example of the information processingunit.

Since the optical disc player 600 comprises the optical disc device 50of the fifth embodiment, it is possible to favorably record or reproduceinformation to or from a multilayer optical disc comprising at leastthree information recording surfaces, and can be applied to a variety ofuses.

Eighth Embodiment

FIG. 16 is a diagram showing the schematic configuration of the opticaldisc recorder in the eighth embodiment of the present invention.

In FIG. 16, the optical disc recorder 700 comprises the optical discdevice 50 of the fifth embodiment, and an encoder 701 for converting theimage information into an information signal for recording on theoptical disc by the optical disc device 50. Desirably, a decoder 702 forconverting the information signal obtained from the optical disc device50 into image information is additionally provided so as to enable thereproduction of the recorded image. The optical disc recorder 700 mayalso comprise an output device 703 such as a cathode-ray tube or liquidcrystal display device for displaying information or a printer forprinting information.

Incidentally, in the eighth embodiment, the optical disc recorder 700corresponds to an example of the information processing device, and theencoder 701 and the decoder 702 correspond to an example of theinformation processing unit.

Since the optical disc recorder 700 comprises the optical disc device 50of the fifth embodiment, it is possible to favorably record or reproduceinformation to or from a multilayer optical disc comprising at leastthree information recording surfaces, and can be applied to a variety ofuses.

Incidentally, the foregoing specific embodiments primarily include theinvention having the following configuration.

The optical head according to one aspect of the present inventioncomprises a first light source for emitting a first laser beam having afirst wavelength λ1, a first planar optical element for transmitting andreflecting the first laser beam at a predetermined ratio, a firstobjective lens for converging the first laser beam onto an informationrecording surface of a first information recording medium, a firstphotodetector for receiving reflected light from the informationrecording surface of the first information recording medium and creatingan information signal and/or an error signal, and a second photodetectorfor receiving transmitted light or reflected light from the first planaroptical element and creating an automatic power control signal forcontrolling an output of the first light source, wherein the firstplanar optical element includes a first surface which the first laserbeam emitted from the first light source enters, and a second surfacefacing the first surface, the first surface and the second surface aremutually parallel, a reflective coat for transmitting and reflecting thefirst laser beam at a predetermined ratio is formed on the firstsurface, and a diffraction grating is formed on the second surface.

According to this configuration, the first light source emits a firstlaser beam having a first wavelength λ1, and the first planar opticalelement transmits and reflects the first laser beam at a predeterminedratio. The first objective lens converges the first laser beam onto aninformation recording surface of a first information recording medium,and the first photodetector receives reflected light from theinformation recording surface of the first information recording medium,and creates an information signal and/or an error signal. The secondphotodetector receives transmitted light or reflected light from thefirst planar optical element, and creates an automatic power controlsignal for controlling the output of the first light source. The firstplanar optical element includes a first surface which the first laserbeam emitted from the first light source enters, and a second surfacefacing the first surface, and the first surface and the second surfaceare mutually parallel. A reflective coat for transmitting and reflectingthe first laser beam at a predetermined ratio is formed on the firstsurface, and a diffraction grating is formed on the second surface.

The first laser beam that enters the first planar optical element passesthrough the second surface, and is internally reflected with the secondsurface and the first surface and subsequently passes through the secondsurface. Here, the first laser beam that passed through the secondsurface without being internally reflected and the first laser beam thatwas internally reflected and passed through the second surface will berespectively diffracted with a different diffraction efficiency based onthe diffraction grating.

Accordingly, even if the first laser beam that enters the first planaroptical element is parallel light, the ±first order diffracted light ofthe first laser beam that passed through the second surface after beinginternally reflected by the first planar optical element will not beparallel to the first laser beam that passes through the second surfacewithout being internally reflected by the first planar optical element.Thus, it is possible to inhibit the interference of the first laser beamin the effective region of the second photodetector for controlling thelaser power of the first light source, and it is thereby possible toaccurately control the laser power of the first laser beam that isemitted from the first light source.

Moreover, with the foregoing optical head, preferably, the first planaroptical element includes a planar polarizing beam splitter. According tothis configuration, a planar polarizing beam splitter can be used as thefirst planar optical element.

Moreover, with the foregoing optical head, preferably, the first planaroptical element includes a planar reflective mirror for reflecting thefirst laser beam so that the first laser beam enters substantiallyperpendicularly the information recording surface of the firstinformation recording medium. According to this configuration, the firstlaser beam can be reflected so that it enters substantiallyperpendicularly the information recording surface of the firstinformation recording medium.

Moreover, with the foregoing optical head, preferably, the firstinformation recording medium includes at least three informationrecording surfaces each having a light transmitting layer different inthickness, and the foregoing optical head further comprises a sphericalaberration correction unit for correcting a spherical aberration thatoccurs according to the thickness of the light transmitting layer of thefirst information recording medium, wherein the spherical aberrationcorrection unit causes diverging light to enter the first objective lensupon recording or reproducing information to or from an informationrecording surface L0 with the thickest light transmitting layer, andcauses converging light to enter the first objective lens upon recordingor reproducing information to or from an information recording surfaceLn with the thinnest light transmitting layer.

According to this configuration, the first information recording mediumincludes at least three information recording surfaces each having alight transmitting layer different in thickness, and the sphericalaberration correction unit corrects a spherical aberration that occursaccording to the thickness of the light transmitting layer of the firstinformation recording medium. In addition, the spherical aberrationcorrection unit causes diverging light to enter the first objective lensupon recording or reproducing information to or from an informationrecording surface L0 with the thickest light transmitting layer, andcauses converging light to enter the first objective lens upon recordingor reproducing information to or from an information recording surfaceLn with the thinnest light transmitting layer.

Accordingly, it is possible to correct the third order sphericalaberration according to the thickness of the light transmitting layer ofthe respective information recording surfaces in the first informationrecording medium including at least three information recording surfaceseach having a light transmitting layer different in thickness.

Moreover, with the foregoing optical head, preferably, a grating depth dof the diffraction grating satisfies (λ1)/6≦d≦(λ1)/4. According to thisconfiguration, since the depth d of the diffraction grating satisfies(λ1)/6≦d≦(λ1)/4, it is possible to inhibit the diffraction efficiency ofthe substantially internally reflected zero order light to anappropriate level.

Moreover, preferably the foregoing optical head further comprises asecond light source for emitting a second laser beam having a secondwavelength λ2 that is larger than the first wavelength λ1, a secondplanar optical element for transmitting and reflecting the second laserbeam at a predetermined ratio and substantially transmitting the firstlaser beam, and a second objective lens for converging the second laserbeam onto an information recording surface of a second informationrecording medium of a type that is different from the first informationrecording medium.

According to this configuration, the second light source emits a secondlaser beam having a second wavelength λ2 that is larger than the firstwavelength λ1, the second planar optical element transmits and reflectsthe second laser beam at a predetermined ratio and substantiallytransmits the first laser beam, and the second objective lens convergesthe second laser beam onto an information recording surface of a secondinformation recording medium of a type that is different from the firstinformation recording medium.

Accordingly, in addition to the first information recording medium,information can be recorded or reproduced to and from a secondinformation recording medium of a type that is different from the firstinformation recording medium.

Moreover, with the foregoing optical head, preferably, the sphericalaberration correction unit causes diverging light or converging light toenter the second objective lens upon recording or reproducinginformation to or from the information recording surface of the secondinformation recording medium.

According to this configuration, the spherical aberration correctionunit causes diverging light or converging light to enter the secondobjective lens upon recording or reproducing information to or from theinformation recording surface of the second information recordingmedium. Accordingly, since the optical axis of the second laser beamthat is headed toward the second photodetector after passing through thesecond planar optical element and the first planar optical element andthe optical axis of the second laser beam that is headed toward thesecond photodetector after being internally reflected two or more timesby the second planar optical element, subsequently entering the firstplanar optical element, and passing through the first planar opticalelement will not mutually be parallel, it is possible to inhibit theinterference of the second laser beam in the effective region of thesecond photodetector.

Moreover, with the foregoing optical head, preferably, zero order lighttransmittance η1 of the first laser beam in the diffraction grating issmaller than zero order light transmittance η2 of the second laser beamin the diffraction grating.

According to this configuration, since the zero order lighttransmittance η1 of the first laser beam in the diffraction grating issmaller than the zero order light transmittance η2 of the second laserbeam in the diffraction grating, it is possible to sufficiently ensurethe detected quantity of light of the second laser beam in the secondphotodetector.

Moreover, with the foregoing optical head, preferably, reflectance ofthe second laser beam on the first surface is less than 5%. According tothis configuration, since the reflectance of the second laser beam onthe first surface is less than 5%, it is possible to sufficiently ensurethe detected quantity of light of the second laser beam in the secondphotodetector. In addition, the quantity of light of the second laserbeam that is headed toward the second photodetector after beinginternally reflected two or more times by the first planar opticalelement will become sufficiently small, and it is possible to inhibitthe interference of the second laser beam in the second photodetector.

Moreover, with the foregoing optical head, preferably, an antireflectioncoat having prescribed reflectance and transmittance for the first laserbeam is formed on the second surface. According to this configuration,since an antireflection coat having prescribed reflectance andtransmittance for the first laser beam is formed on the second surface,it is possible to inhibit the first laser beam from being internallyreflected by the first planar optical element.

Moreover, with the foregoing optical head, preferably, reflectance R1for the first laser beam in the antireflection coat and reflectance R2for the second laser beam in the antireflection coat satisfy R1<(R2)/2.

According to this configuration, since the reflectance R1 for the firstlaser beam in the antireflection coat and the reflectance R2 for thesecond laser beam in the antireflection coat satisfy R1<(R2)/2, it ispossible to inhibit the first laser beam from being internally reflectedby the first planar optical element.

Moreover, preferably, the foregoing optical head further comprises athird light source for emitting a third laser beam having a thirdwavelength λ3 that is larger than the second wavelength λ2, wherein thesecond objective lens converges the third laser beam onto an informationrecording surface of a third information recording medium of a type thatis different from the first information recording medium and the secondinformation recording medium, and the second planar optical elementtransmits and reflects the third laser beam at a predetermined ratio.

According to this configuration, the third light source emits a thirdlaser beam having a third wavelength λ3 that is larger than the secondwavelength λ2, the second objective lens converges the third laser beamonto an information recording surface of a third information recordingmedium of a type that is different from the first information recordingmedium and the second information recording medium, and the secondplanar optical element transmits and reflects the third laser beam at apredetermined ratio.

Accordingly, in addition to the first information recording medium andthe second information recording medium, it is possible to record andreproduce information to and from a third information recording mediumof a type that is different from the first information recording mediumand the second information recording medium.

Moreover, with the foregoing optical head, preferably, the sphericalaberration correction unit causes converging light to enter the secondobjective lens upon recording or reproducing information to or from theinformation recording surface of the second information recordingmedium, and causes diverging light to enter the second objective lensupon recording or reproducing information to or from the informationrecording surface of the third information recording medium.

According to this configuration, the spherical aberration correctionunit causes converging light to enter the second objective lens uponrecording or reproducing information to or from the informationrecording surface of the second information recording medium, and causesdiverging light to enter the second objective lens upon recording orreproducing information to or from the information recording surface ofthe third information recording medium.

Accordingly, since the optical axis of the second laser beam or thethird laser beam that is headed toward the second photodetector afterpassing through the second planar optical element and the first planaroptical element and the optical axis of the second laser beam or thethird laser beam that is headed toward the second photodetector afterbeing internally reflected two or more times by the second planaroptical element, subsequently entering the first planar optical elementand passing through the first planar optical element will not mutuallybe parallel, it is possible to inhibit the interference of the secondlaser beam or the third laser beam in the effective region of the secondphotodetector.

Moreover, with the foregoing optical head, preferably, an antireflectioncoat having prescribed reflectance and transmittance for the first laserbeam, the second laser beam and the third laser beam is formed on thesecond surface.

According to this configuration, since an antireflection coat havingprescribed reflectance and transmittance for the first laser beam, thesecond laser beam and the third laser beam is formed on the secondsurface, it is possible to inhibit the first laser beam, the secondlaser beam and the third laser beam from being internally reflected bythe first planar optical element.

Moreover, with the foregoing optical head, preferably, the diffractiongrating has a concave and convex pattern in which a plurality ofstraight lines parallel to an incident plane of the first laser beam arealigned in parallel. According to this configuration, the first laserbeam can be diffracted using a diffraction grating having a concave andconvex pattern in which a plurality of straight lines parallel to anincident plane of the first laser beam are aligned in parallel.

Moreover, with the foregoing optical head, preferably, the diffractiongrating has an annular concave and convex pattern around an incidentlight axis of the first laser beam. According to this configuration, thefirst laser beam can be diffracted using a diffraction grating having anannular concave and convex pattern around an incident light axis of thefirst laser beam.

Moreover, with the foregoing optical head, preferably, the diffractiongrating is a binary diffraction grating. According to thisconfiguration, since the diffraction grating can be formed easily with aprocess such as etching, it can be created inexpensively.

The optical head according to another aspect of the present inventioncomprises a first light source for emitting a first laser beam having afirst wavelength λ1, a second light source for emitting a second laserbeam having a second wavelength λ2 that is larger than the firstwavelength λ1, a first planar optical element for transmitting andreflecting the first laser beam at a predetermined ratio, a secondplanar optical element for transmitting and reflecting the second laserbeam at a predetermined ratio and substantially transmitting the firstlaser beam, a first objective lens for converging the first laser beamonto an information recording surface of a first information recordingmedium, a second objective lens for converging the second laser beamonto an information recording surface of a second information recordingmedium of a type that is different from the first information recordingmedium, a first photodetector for receiving reflected light from theinformation recording surface of the first information recording mediumand creating an information signal and/or an error signal, and a secondphotodetector for receiving transmitted light or reflected light fromthe first planar optical element and creating an automatic power controlsignal for controlling an output of the first light source, wherein thefirst planar optical element includes a first surface which the firstlaser beam emitted from the first light source enters, and a secondsurface facing the first surface, the first surface and the secondsurface are mutually parallel, a reflective coat for transmitting andreflecting the first laser beam at a predetermined ratio is formed onthe first surface, and an antireflection coat having prescribedreflectance and transmittance for the first laser beam is formed on thesecond surface.

According to this configuration, the first light source emits a firstlaser beam having a first wavelength λ1, and the second light sourceemits a second laser beam having a second wavelength λ2 that is largerthan the first wavelength λ1. The first planar optical element transmitsand reflects the first laser beam at a predetermined ratio, and thesecond planar optical element transmits and reflects the second laserbeam at a predetermined ratio and substantially transmits the firstlaser beam. The first objective lens converges the first laser beam ontoan information recording surface of a first information recordingmedium, and the second objective lens converges the second laser beamonto an information recording surface of a second information recordingmedium of a type that is different from the first information recordingmedium. The first photodetector receives reflected light from theinformation recording surface of the first information recording mediumand creating an information signal and/or an error signal. The secondphotodetector receives transmitted light or reflected light from thefirst planar optical element and creates an automatic power controlsignal for controlling an output of the first light source. The firstplanar optical element includes a first surface which the first laserbeam emitted from the first light source enters, and a second surfacefacing the first surface, and the first surface and the second surfaceare mutually parallel. A reflective coat for transmitting and reflectingthe first laser beam at a predetermined ratio is formed on the firstsurface, and an antireflection coat having prescribed reflectance andtransmittance for the first laser beam is formed on the second surface.

The first laser beam that entered the first planar optical elementpasses through the second surface. Here, since an antireflection coat isformed on the second surface, the internal reflection of the first laserbeam is inhibited, and the quantity of light of the first laser beamthat is headed toward the second photodetector after being internallyreflected at least two or more times by the first planar optical elementcan be made sufficiently small.

Accordingly, even if the first laser beam that enters the first planaroptical element is parallel light, since the first laser beam passesthrough the second surface without being substantially internallyreflected by the first planar optical element, it is possible to inhibitthe interference of the first laser beam in the effective region of thesecond photodetector for controlling the laser power of the first lightsource, and it is thereby possible to accurately control the laser powerof the first laser beam that is emitted from the first light source.

Moreover, with the foregoing optical head, preferably, reflectance R1for the first laser beam in the antireflection coat is smaller than0.01.

According to this configuration, since the reflectance R1 for the firstlaser beam in the antireflection coat is smaller than 0.01, the quantityof light of the first laser beam that is headed toward the secondphotodetector after being internally reflected at least two or moretimes by the first planar optical element can be made sufficientlysmall.

The optical element with a diffraction grating according to anotheraspect of the present invention is an optical element with a diffractiongrating for transmitting and reflecting a laser beam having apredetermined wavelength λ at a predetermined ratio, and guiding thelaser beam to a photodetector that creates an automatic power controlsignal for controlling an output of a light source that emits the laserbeam, comprising a first surface which the laser beam enters, and asecond surface facing the first surface, wherein the first surface andthe second surface are mutually parallel, a diffraction grating isformed on the second surface, a grating depth d of the diffractiongrating satisfies λ/6≦d≦λ/4, and a pitch p of the diffraction gratingsatisfies p>λ.

According to this configuration, the optical element with a diffractiongrating transmits and reflects a laser beam having a predeterminedwavelength λ at a predetermined ratio, and guides the laser beam to aphotodetector that creates an automatic power control signal forcontrolling an output of a light source that emits the laser beam. Theoptical element with a diffraction grating comprises a first surfacewhich the laser beam enters, and a second surface facing the firstsurface, wherein the first surface and the second surface are mutuallyparallel. A diffraction grating is formed on the second surface. Agrating depth d of the diffraction grating satisfies λ/6≦d≦λ/4, and apitch p of the diffraction grating satisfies p>λ.

The first laser beam that enters the optical element with a diffractiongrating passes through the second surface, and is internally reflectedwith the second surface and the first surface and subsequently passesthrough the second surface. Here, the first laser beam that passedthrough the second surface without being internally reflected and thefirst laser beam that was internally reflected and passed through thesecond surface will be respectively diffracted with a differentdiffraction efficiency based on the diffraction grating.

Accordingly, even if the laser beam that enters the optical element witha diffraction grating is parallel light, the laser beam that passedthrough the second surface after being internally reflected in theoptical element with a diffraction grating will not be parallel to thelaser beam that passes through the second surface without beinginternally reflected in the optical element with a diffraction grating.Thus, it is possible to inhibit the interference of the laser beam inthe effective region of the photodetector for controlling the laserpower of the light source, and it is thereby possible to accuratelycontrol the laser power of the laser beam that is emitted from the lightsource.

Moreover, with the foregoing optical element with a diffraction grating,preferably, a reflective coat for transmitting and reflecting the laserbeam at a predetermined ratio is formed on the first surface.

According to this configuration, since a reflective coat fortransmitting and reflecting the laser beam at a predetermined ratio isformed on the first surface, either the transmitted light or thereflected light can be guided to the information recording medium, andthe other can be guided to the photodetector.

Moreover, with the foregoing optical element with a diffraction grating,preferably, an angle of incidence θ of the laser beam that enters thefirst surface is 45±10 [deg]. According to this configuration, since anangle of incidence θ of the laser beam that enters the first surface is45±10 [deg], the laser beam can be reflected so that it enters theinformation recording surface of the information recording mediumsubstantially perpendicularly on the first surface.

The optical disc device according to another aspect of the presentinvention comprises the optical head according to any one of theforegoing paragraphs, a motor for rotatively driving an informationrecording medium, and a control unit for controlling the optical headand the motor. According to this configuration, the foregoing opticalhead can be applied to the optical disc device.

The information processing device according to another aspect of thepresent invention comprises the optical disc device according to theforegoing paragraph, and an information processing unit for processinginformation recorded in the optical disc device and/or informationreproduced from the optical disc device. According to thisconfiguration, the optical disc device comprising the foregoing opticalhead can be applied to the information processing device.

The specific embodiments and examples that were described above arefirst and foremost for clarifying the technical contents of the presentinvention, and the present invention should not be narrowly interpretedby being limited such specific examples, and may be variously modifiedand worked within the scope of the spirit and claimed subject matter ofthe present invention.

The optical head, the optical disc device and the optical element with adiffraction grating according to the present invention are able todetect an APC signal with an inexpensive configuration using a planaroptical element in a multilayer high density optical disc including atleast three information recording surfaces, and the present invention isuseful as an optical head for recording or reproducing information to orfrom an information recording medium such as an optical disc, an opticaldisc device comprising the foregoing optical head, and an opticalelement with a diffraction grating for guiding the transmitted light orthe reflected light to a photodetector that creates an automatic powercontrol signal for controlling an output of a light source.

In addition, the information processing device comprising the opticaldisc device according to the present invention is able to favorablyrecord or reproduce information to and from a multilayer high densityoptical disc including at least three information recording surfaces,and can be applied to a wide variety of uses.

The invention claimed is:
 1. An optical head, comprising: a first lightsource for emitting a first laser beam having a first wavelength λ1; asecond light source for emitting a second laser beam having a secondwavelength λ2 that is larger than the first wavelength λ1; a firstplanar optical element for transmitting and reflecting the first laserbeam at a predetermined ratio; a second planar optical element disposedbetween the second light source and the first planar optical element,for transmitting and reflecting the second laser beam at a predeterminedratio and substantially transmitting the first laser beam; a firstobjective lens for converging the first laser beam reflected by thefirst planar optical element onto an information recording surface of afirst information recording medium; a second objective lens forconverging the second laser beam reflected by the second planar opticalelement onto an information recording surface of a second informationrecording medium of a type that is different from the first informationrecording medium; a first photodetector for receiving reflected lightfrom the information recording surface of the first informationrecording medium and creating an information signal or an error signal;and a second photodetector for receiving transmitted light from thefirst planar optical element and creating an automatic power controlsignal for controlling an output of the first light source, wherein thefirst planar optical element includes a first surface which the firstlaser beam emitted from the first light source enters, and a secondsurface facing the first surface, the first surface and the secondsurface are mutually parallel, a reflective coat for transmitting andreflecting the first laser beam at a predetermined ratio is formed onthe first surface, and an antireflection coat having prescribedreflectance and transmittance for the first laser beam is formed on thesecond surface.
 2. The optical head according to claim 1, wherein thefirst planar optical element includes a planar reflective mirror forreflecting the first laser beam so that the first laser beam enterssubstantially perpendicularly the information recording surface of thefirst information recording medium.
 3. The optical head according toclaim 1, wherein the first information recording medium includes atleast three information recording surfaces each having a lighttransmitting layer different in thickness, the optical head furthercomprises a spherical aberration correction unit for correcting aspherical aberration that occurs according to a thickness of the lighttransmitting layer of the first information recording medium, and thespherical aberration correction unit causes diverging light to enter thefirst objective lens upon recording or reproducing information to orfrom an information recording surface L0 with a thickest lighttransmitting layer, and causes converging light to enter the firstobjective lens upon recording or reproducing information to or from aninformation recording surface Ln with a thinnest light transmittinglayer.
 4. The optical head according to claim 1, wherein the firstinformation recording medium includes at least three informationrecording surfaces each having a light transmitting layer different inthickness, the optical head further comprises a spherical aberrationcorrection unit for correcting a spherical aberration that occursaccording to a thickness of the light transmitting layer of the firstinformation recording medium, and the spherical aberration correctionunit causes diverging light or converging light to enter the secondobjective lens upon recording or reproducing information to or from theinformation recording surface of the second information recordingmedium.
 5. The optical head according to claim 1, further comprising: athird light source for emitting a third laser beam having a thirdwavelength λ3 that is larger than the second wavelength λ2, wherein thesecond objective lens converges the third laser beam onto an informationrecording surface of a third information recording medium of a type thatis different from the first information recording medium and the secondinformation recording medium, and the second planar optical elementtransmits and reflects the third laser beam at a predetermined ratio. 6.The optical head according to claim 5, wherein the first informationrecording medium includes at least three information recording surfaceseach having a light transmitting layer different in thickness, theoptical head further comprises a spherical aberration correction unitfor correcting a spherical aberration that occurs according to athickness of the light transmitting layer of the first informationrecording medium, and the spherical aberration correction unit causesconverging light to enter the second objective lens upon recording orreproducing information to or from the information recording surface ofthe second information recording medium, and causes diverging light toenter the second objective lens upon recording or reproducinginformation to or from the information recording surface of the thirdinformation recording medium.
 7. The optical head according to claim 1,wherein reflectance R1 for the first laser beam in the antireflectioncoat is smaller than 0.01.
 8. An optical disc device, comprising: theoptical head according to claim 1; a motor for rotatively driving aninformation recording medium; and a control unit for controlling theoptical head and the motor.
 9. An information processing device,comprising: the optical disc device according to claim 8; and aninformation processing unit for processing information recorded in theoptical disc device or information reproduced from the optical discdevice.
 10. An optical head, comprising: a first light source foremitting a first laser beam having a first wavelength λ1; a first planaroptical element for transmitting and reflecting the first laser beam ata predetermined ratio; a first objective lens for converging the firstlaser beam onto an information recording surface of a first informationrecording medium; a first photodetector for receiving reflected lightfrom the information recording surface of the first informationrecording medium and creating an information signal or an error signal;and a second photodetector for receiving transmitted light from thefirst planar optical element and creating an automatic power controlsignal for controlling an output of the first light source, wherein thefirst planar optical element includes a first surface which the firstlaser beam emitted from the first light source enters, and a secondsurface facing the first surface, the first surface and the secondsurface are mutually parallel, a reflective coat for transmitting andreflecting the first laser beam at a predetermined ratio is formed onthe first surface, and a diffraction grating is formed on the secondsurface, and wherein the first information recording medium includes atleast three information recording surfaces each having a lighttransmitting layer different in thickness, the optical head furthercomprises a spherical aberration correction unit for correcting aspherical aberration that occurs according to a thickness of the lighttransmitting layer of the first information recording medium, and thespherical aberration correction unit causes diverging light to enter thefirst objective lens upon recording or reproducing information to orfrom an information recording surface L0 with a thickest lighttransmitting layer, and causes converging light to enter the firstobjective lens upon recording or reproducing information to or from aninformation recording surface Ln with a thinnest light transmittinglayer, and a grating depth d of the diffraction grating satisfies:(λ1)/6≦d≦(λ1)/4.
 11. The optical head according to claim 10, wherein thefirst planar optical element includes a planar polarizing beam splitter.12. The optical head according to claim 10, wherein the first planaroptical element includes a planar reflective mirror for reflecting thefirst laser beam so that the first laser beam enters substantiallyperpendicularly the information recording surface of the firstinformation recording medium.
 13. The optical head according to claim10, wherein the diffraction grating has a concave and convex pattern inwhich a plurality of straight lines parallel to an incident plane of thefirst laser beam are aligned in parallel.
 14. The optical head accordingto claim 13, wherein the diffraction grating is a binary diffractiongrating.
 15. The optical head according to claim 10, wherein thediffraction grating has an annular concave and convex pattern around anincident light axis of the first laser beam.
 16. An optical head,comprising: a first light source for emitting a first laser beam havinga first wavelength λ1; a second light source for emitting a second laserbeam having a second wavelength λ2 that is larger than the firstwavelength λ1; a first planar optical element for transmitting andreflecting the first laser beam at a predetermined ratio; a secondplanar optical element for transmitting and reflecting the second laserbeam at a predetermined ratio and substantially transmitting the firstlaser beam; a first objective lens for converging the first laser beamonto an information recording surface of a first information recordingmedium; a second objective lens for converging the second laser beamonto an information recording surface of a second information recordingmedium of a type that is different from the first information recordingmedium; a first photodetector for receiving reflected light from theinformation recording surface of the first information recording mediumand creating an information signal or an error signal; and a secondphotodetector for receiving transmitted light from the first planaroptical element and creating an automatic power control signal forcontrolling an output of the first light source, wherein the firstplanar optical element includes a first surface which the first laserbeam emitted from the first light source enters, and a second surfacefacing the first surface, the first surface and the second surface aremutually parallel, a reflective coat for transmitting and reflecting thefirst laser beam at a predetermined ratio is formed on the firstsurface, and a diffraction grating is formed on the second surface. 17.The optical head according to claim 16, wherein the first informationrecording medium includes at least three information recording surfaceseach having a light transmitting layer different in thickness, theoptical head further comprises a spherical aberration correction unitfor correcting a spherical aberration that occurs according to athickness of the light transmitting layer of the first informationrecording medium, and the spherical aberration correction unit causesdiverging light or converging light to enter the second objective lensupon recording or reproducing information to or from the informationrecording surface of the second information recording medium.
 18. Theoptical head according to claim 16, wherein zero order lighttransmittance η1 of the first laser beam in the diffraction grating issmaller than zero order light transmittance η2 of the second laser beamin the diffraction grating.
 19. The optical head according to claim 16,wherein reflectance of the second laser beam on the first surface isless than 5%.
 20. The optical head according to claim 16, wherein anantireflection coat having prescribed reflectance and transmittance forthe first laser beam is formed on the second surface.
 21. The opticalhead according to claim 20, wherein reflectance R1 for the first laserbeam in the antireflection coat and reflectance R2 for the second laserbeam in the antireflection coat satisfy:R1<(R2)/2.
 22. The optical head according to claim 16, furthercomprising: a third light source for emitting a third laser beam havinga third wavelength λ3 that is larger than the second wavelength λ2,wherein the second objective lens converges the third laser beam onto aninformation recording surface of a third information recording medium ofa type that is different from the first information recording medium andthe second information recording medium, and the second planar opticalelement transmits and reflects the third laser beam at a predeterminedratio.
 23. The optical head according to claim 22, wherein the firstinformation recording medium includes at least three informationrecording surfaces each having a light transmitting layer different inthickness, the optical head further comprises a spherical aberrationcorrection unit for correcting a spherical aberration that occursaccording to a thickness of the light transmitting layer of the firstinformation recording medium, and the spherical aberration correctionunit causes converging light to enter the second objective lens uponrecording or reproducing information to or from the informationrecording surface of the second information recording medium, and causesdiverging light to enter the second objective lens upon recording orreproducing information to or from the information recording surface ofthe third information recording medium.
 24. The optical head accordingto claim 22, wherein an antireflection coat having prescribedreflectance and transmittance for the first laser beam, the second laserbeam and the third laser beam is formed on the second surface.
 25. Anoptical element with a diffraction grating for transmitting andreflecting, at a predetermined ratio, a laser beam having apredetermined wavelength λ, and guiding the laser beam to aphotodetector that creates an automatic power control signal forcontrolling an output of a light source that emits the laser beam, theoptical element comprising: a first surface which the laser beam enters;and a second surface facing the first surface, wherein the first surfaceand the second surface are mutually parallel, a diffraction grating isformed on the second surface, a grating depth d of the diffractiongrating satisfies:λ/6≦d≦λ/4, and a pitch p of the diffraction grating satisfies:p>λ.
 26. The optical element with a diffraction grating according toclaim 25, wherein a reflective coat for transmitting and reflecting thelaser beam at a predetermined ratio is formed on the first surface. 27.The optical element with a diffraction grating according to claim 25,wherein an angle of incidence θ of the laser beam that enters the firstsurface is 45 degrees±10 degrees.